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PLATE  I. 


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ELECTEIC   POWEE 
TEANSMIS8I0N 


A  PRACTICAL   TREATISE 
FOR  PRACTICAL  MEN 


LOUIS  BELL,  Ph.  D. 

MEMBEB  AMERICAN  INSTITUTE  OF  ELECTRICAL  ENOINEEBS 


FOURTH  EDITION 
REVISED  AND  ENLARGED 


NEW   YORK 

McGRAW   PUBLISHING   CO. 
1006 


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GOPYBIOHT,  1906,  BY 

McGRAW  PUBLISHING  CO. 
New  York 


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94186 

MA:   r  1906 


^u-iw^h 


•3 


PREFACE   TO   FIRST   EDITION. 


This  volume  is  designed  to  set  forth  in  the  simplest  possible 
manner,  the  fundamental  facts  concerning  present  practice  in 
electrical  power  transmission. 

Busy  men  have  little  time  txy  spend  in  discussing  theories  of 
which  the  practical  results  are  known,  or  in  following  the 
derivation  of  formula)  which  no  one  disputes.  The  author  has 
therefore  endeavored,  in  introducing  such  theoretical  consid- 
erations as  are  necessary,  to  explain  them  in  the  most  direct 
way  practicable;  using  proximate  methods  of  proof  when  pre- 
cise and  general  ones  would  lead  to  mathematical  complica- 
tions without  altering  the  conclusion  for  the  purpose  in  hand, 
and  stating  only  the  results  of  investigation  when  the  processes 
are  undesfrably  complicated. 

In  writing  of  a  many-sided  and  rapidly  changing  art,  it  is 
impossible  in  a  finite  compass  to  cover  all  the  phases  of  the 
subject  or  to  prophesy  the  modifications  that  time  will  bring 
forth;  hence,  the  epoch  of  this  work  is  the  present  and  the 
point  of  view  chosen  is  that  of  the  man,  engineer  or  not,  who 
desires  to  know  what  can  be  accomplished  by  electrical  power 
transmission,  and  by  what  processes  the  work  is  planned  and 
carried  out.  This  treatment  is  not  without  value  to  the  stu- 
dent who  wishes  to  couple  his  investigations  of  electrical 
theory  with  its  application  in  the  hands  of  engineers,  and  puts 
the  facts  regarding  a  very  great  and  important  development  of 
applied  electricity  in  the  possession  of  the  general  reader. 

Such  apparatus  as  is  described  is  intended  to  be  typical  of 
the  methods  used,  rather  than  representative  of  any  particular 
scheme  of  manufacture  or  fashion  in  design.  These  last 
change  almost  from  month  to  month,  while  the  general  con- 
ditions remain  fairly  stable,  and  the  underlying  principles  are 
of  permanent  value. 

Janiuiry,  1897, 


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PREFACE   TO  FOURTH   EDITION. 


In  the  three  years  which  have  elapsed  since  the  third  edition 
of  this  work  went  to  press,  there  have  been  very  few  sensational 
changes  in  electric  power  transmission.  Plants  have  multiplied 
and  higher  voltages  have  become  common  without  any  radical 
innovations  in  general  practice.  There  has  been  very  con- 
siderable improvement  in  station  accessories  and  in  details  of 
construction,  especially  of  the  line.  The  resources  of  the  art 
with  respect  to  distribution  of  energy  for  various  purposes  are 
steadily  growing  richer,  much  apparatus  special  a  few  years 
since  has  become  standardized,  and  altogether  the  aggregate 
of  minor  changes  has  made  a  new  edition  imperative.  It  has 
been  needful  to  devote  some  special  attention  to  the  important 
accessory  apparatus  of  which  modern  stations  are  full,  and  to 
make  use  of  considerable  new  material  of  a  more  general  sort, 
as  well  as  to  eliminate  some  descriptive  matter  which  had  to 
do  with  things  which  are  obsolete  and  without  historical  im- 
portance. It  has  now  become  a  hopeless  task  to  keep  track 
of  10,000  volt  plants  which  are  at  present  utterly  commonplace, 
so  that  in  retaining  merely  as  a  matter  of  general  interest,  a  list 
of  high  voltage  transmission  plants,  nothing  under  20,000  volts 
is  included.  This  fact  expresses  better  than  words  the  trend 
of  recent  advances  in  the  art. 

September  J  1905. 


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CONTENTS. 


Craptbr.  pAoa. 

I.    Elementary  Principles 1 

II.    General  Conditions  op  Power  Transmission     ...  23 

III.   Power  Transmission  by  Continuous  Currents  ...  77 

IV.   Some  Properties  op  Alternating  Circuits     ....  125 

V.   Power  Transmission  by  Alternating  Currents   .    .  158 

VI.   Alternating  Current  Motors 217 

VII.  Current  Reorganizers 280 

VIII.  Engines  and  Boilers      309 

IX.   Water-Wheels      349 

X.   Hydraulic  Development 387 

XI.   The  Organization  op  a  Power  Station 418 

XII.  Auxiliary  and  Switchboard  Apparatus 455 

XIII.  The  Line 474 

XIV.  Line  Construction 536 

XV.  Methods  op  Distribution 581 

XVI.   The  Commercial  Problem 639 

XVII.  The  Measurement  of  Electrical  Energy 660 

XVIII.  High  Voltage  Transmission 687 


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LIST  OF  TABLES. 


Efficdsnct  of  Wire  Rope  Drives 39 

Wire  Ropes  and  Pullets  Therefor 42 

Loss  OF  Head  in  Hydraulic  Pipes 47 

Loss  OF  Air  Pressure  in  Pipes 52 

Efficiencies  of  Electric  Motors 62 

Efficiencies  of  Electric  and  Other  Transmissions     ....  74 

Performance  of  Small  Polyphase  Motors 270 

&rEAM  Consumption  of  Enoines 318 

EvAPORATiyB  Power  of  Fuels 329 

Evaporative  Tests  of  Boilers *  .   .   .  330 

Coal  Consumption  of  Engines 332 

Table  for  Weirs 391 

Properties  of  Steel  Hydraulic  Pipe 408 

Properties  of  Copper  and  Other  Wires 486 

Size,  Resistance,  and  Weiohts  of  Copper  Wires 509 

Natural  Tangents,  Sines,  and  Cosines 522 

Size,  Weight,  and  Tensile  Strength  of  Line  Wires  ....  539 

Sizes  and  Weights  of  Wooden  Poles 551 

Tensile  Strength  of  Woods 554 

Properties  of  Direct  and  Alternating  Current  Arcs  .   .   .  592 

Cost  of  Power  with  Various  Engines 640 

Cost  of  Intermittent  Power  with  Various  Engines   ....  643 

Cost  of  Electric  Motors 644 

Typical  List  of  Discoxtntb 657 

List  of  American  Transmission  at  or  above  20,000  Voi/rs    .  702 


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ELECTRIC  TRANSMISSION  OF  POWER. 


CHAPTER  I. 

ELEMENTARY  PRINCIPLES. 

It  has  long  been  the  fashion  to  speak  of  what  we  are 
pleased  to  call  electricity  as  a  mysterious  "force,"  and  to 
attribute  to  everything  connected  with  it  occult  characteristics 
better  suited  to  mediaeval  wizardry  than  to  modem  science. 
This  unhappy  condition  of  affairs  has,  in  the  main,  come  about 
through  the  indistinctness  of  some  of  our  fundamental  ideas 
and  inexactitude  in  expressing  them. 

To  speak  specifically,  there  has  been,  even  hi  the  minds 
and  writings  of  some  who  ought  to  know  better,  a  tendency 
toward  confusing  the  extremely  hazy  individuality  of  *' elec- 
tricity" with  the  sharply  defined  properties  of  electrical 
energy.  We  have  been  so  overrun  by  theories  of  electricity, 
two-fluid,  one-fluid,  and  non-fluid  —  by  electrically  ** charged" 
atoms  and  duplex  ethers,  that  we  have  well-nigh  forgotten  the 
very  great  uncertainty  as  to  the  concrete  existence  of  elec- 
tricity itself.  Even  admitting  it  to  be  an  entity,  it  most 
assuredly  is  not  a  force,  mysterious  or  otherwise.  Electrical 
force  there  is,  and  electrical  energy  there  is,  and  with  them 
we  can  freely  experiment,  but  for  most  practical  purposes 
"electricity"  is  merely  the  numerical  factor  connecting  the 
two.  It  is  related  to  electrical  energy  much  as  that  other 
hypothetical  fluid  ''caloric"  was  supposed  to  be  related  to 
heat  energy.  The  analogy  is  not  absolutely  exact,  but  it 
nevertheless  summarizes  the  real  facts  in  the  case. 

The  day  has  passed  wherein  we  were  at  liberty  to  think  of 
"electricity"  as  flowing  through  a  material  tube  or  as  plas- 


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2  ELECTRIC  TRANSMISSION  OF  POWER. 

tered  upon  bodies  like  a  coat  of  paint.  The  things  with  ^vhich 
we  have  now  to  deal  are  the  various  factors  of  electrical 
energy. 

It  is  the  purpose  of  this  chapter  to  treat  of  that  form  of 
energy  which  we  denominate  electrical,  to  discuss  its  relation 
to  other  forms  of  energy  and  some  of  the  transformations 
which  they  may  reciprocally  imdergo. 

Speaking  broadly,  energy  is  power  of  doing  work.  The 
energy  of  a  body  at  any  moment  represents  its  inherent  capacity 
for  doing  work  of  some  sort  on  other  bodies.  This,  however, 
must  not  be  understood  as  implying  that  the  aforesaid  energy 
is  limited  by  our  power  of  utilizing  it.  We  may  or  may  not  be 
able  to  employ  it  to  advantage  or  xmder  possible  conditions. 
As  an  example,  take  the  massive  weight  of  a  pile  driver. 
Raised  to  its  full  height  it  possesses  a  certain  amount  of  gravi- 
tational energy  —  a  possibility  of  doing  useful  work.  This 
energy  is  temporarily  unemployed  and  appears  only  as  a  stress 
on  the  supporting  rope  and  frame-work.  Under  these  cir- 
cumstances, wherein  the  energy  exists  in  static  form,  it  is 
generally  known  as  potential  energy. 

Now  let  the  weight  fall  and  with  swiftly  gathering  velocity 
it  strikes  the  pile  and  does  work  upon  it,  settling  it  deep  into 
the  mud.  The  energy  due  to  the  blow  of  the  moiring  weighty 
energy  of  motion  in  other  words,  is  called  kinetic.  But  at  the 
bottom  of  its  fall  the  weight  still  has  potential  energy  with 
reference  to  points  below  it,  and  we  realize  this  as  the  pile 
settles  lower  and  each  successive  blow  becomes  more  forceful. 
At  some  point  we  are  unable  further  to  utilize  the  fall,  and 
have  then  reached  the  limit  of  the  available  energy  in  this  par- 
ticular case. 

We  must  not  forget,  however,  that  each  time  the  weight 
was  lifted,  work  had  to  be  done  against  gravitation  to  give  the 
weight  its  point  of  vantage  with  respect  to  available  energy. 
This  work  was  probably  done  by  utilizing  the  energy  of 
expanding  steam  —  in  other  words,  the  energy  of  the  steam 
was  transformed  through  doing  work  on  the  piston  into  kinetic 
energy  of  the  latter,  which,  through  doing  work  against  gravi- 
tation, has  been  enabled  again  to  reappear  as  the  energy  of 
a  falling  body,  and  to  do  work  on  the  driven  pile.     And  back 


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ELEMENTARY  PRINCIPLES,  8 

of  the  steam  energy  is  the  heat  energy,  by  which  work  is 
done  on  the  water  in  the  boiler,  and  yet  back  of  this  the  chemi- 
cal energy  of  the  coal,  transformed  into  heat  energy  and  doing 
work  on  the  minute  particles  of  iron  in  the  boiler,  for  we  know 
that  heat  is  a  species  of  kinetic  energy. 

Even  the  work  done  on  our  pile  is  not  permitted  to  go  un- 
transformed  into  energy.  Part  is  transformed  into  heat  energy 
through  friction  and  compression  of  the  pile,  part  through  fric- 
tion of  the  water,  and  part  raises  ripples  that  may  lift  against 
gravity  chips  and  pebbles  on  a  neighboring  shore.  Other  frac- 
tions go  into  the  vibrational  energy  of  sound ;  into  heating  the 
weight  so  that  it  gives  out  warmth  —  radiant  energy  —  to  the 
hand  when  held  near  it  and  to  the  surrounding  air;  and  into 
electrical  work  done  on  the  weight  and  neighboring  objects,  for 
the  weight  unquestionably  receives  a  minute  amount  of  elec- 
trical energy  at  each  blow.  Thus,  a  comparatively  simple 
mechanical  process  involves  a  long  series  of  transformations 
of  energy. 

No  energy  is  ever  created  or  destroyed,  it  merely  is  changed 
in  form  to  reappear  elsewhere,  and  work  done  is  the  link 
between  one  form  of  energy  and  another.  And  we  may  lay 
down  another  law  of  almost  as  serious  import:  No  form  of 
energy  is  ever  transformed  completely  into  any  other. 

On  the  contrary,  the  general  rule  is  that  with  each  transfor- 
mation several  kinds  of  energy  appear  in  varying  amounts,  and 
among  them  we  may  always  reckon  heat.  The  object  of  any 
transformation  is  usually  a  single  form  of  energy,  hence  practi- 
cally no  such  thing  as  perfectly  efficient  transformation  can  be 
obtained.  The  energy  by-products  for  the  most  part  cannot 
be  utilized  and  are  frittered  away  in  useless  work  or  in  storing 
up  kinds  of  potential  energy  that  cannot  be  employed. 

The  greatest  loss  is  in  heat,  which  is  dissipated  in  various 
ways  and  cannot  be  recovered.  The  presence  of  unutilized 
heat  always  denotes  waste  of  energy. 

From  what  has  gone  before,  we  can  readily  appreciate  that 
when  we  do  work  with  the  object  of  rendering  available  a 
particular  kind  of  energy,  the  method  must  be  intelligently 
selected,  else  there  will  result  useless  by-products  of  energy 
which  will  seriously  lower  the  efficiency  of  the  operation. 


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4  ELECTRIC  TRANSMISSION  OF  POWER, 

Whenever  possible  we  utilize  potential  energy  already  exist- 
ing in  securing  a  transformation.  Thus  if  heat  is  wanted,  the 
easiest  way  of  getting  it  is  to  bum  coal,  and  to  allow  its  energy 
to  become  kinetic  as  heat.  If  we  want  mechanical  work  done, 
we  set  heat  energy  to  work  in  the  most  efficient  way  practi- 
cable. If  electrical  energy  is  desired,  we  set  the  energy  of 
st^am  to  revolving  the  armature  of  a  dynamo.  If  the  right 
method  of  transformation  is  not  chosen,  much  of  the  energy 
will  turn  up  in  forms  that  we  do  not  want  or  cannot  utilize. 
Burning  coal  is  a  very  bad  way  of  getting  sound,  just  as  play- 
ing a  comet  is  but  a  poor  means  of  getting  heat,  although  a 
fire  does  produce  a  trifling  amount  of  soimd,  and  a  comet  by 
continual  vibration  must  be  warmed  to  a  minute  degree. 

These  seem,  and  perhaps  are,  extreme  instances,  but  when 
we  realize  that,  somewhat  to  the  discredit  of  human  ingenuity, 
less  than  one-twentieth  of  the  electrical  energy  supplied  to  an 
incandescent  lamp  appears  in  the  form  of  light,  the  comparison 
becomes  grimly  suggestive. 

Understanding  now  that  in  order  to  obtain  energy  in  any 
given  form  (such  as  electrical),  particular  methods  of  transfor- 
mation must  be  used  in  order  to  secure  anything  like  efficiency, 
we  may  look  a  little  more  closely  at  various  types  of  energy  to 
discover  the  characteristics  that  may  indicate  efficient  methods 
of  transformation,  particularly  as  regards  electrical  energy. 

Speaking  broadly,  one  may  divide  energy  into  three  classes: 

1st.  Those  forms  of  energy  which  have  to  do  with  move- 
ments of,  or  strains  in,  masses  of  matter.  In  this  class  may 
be  included  the  ordinary  forms  of  kinetic  energy  of  moving 
bodies  and  the  like. 

2d.  Those  which  are  concerned  with  movements  of,  or 
strains  in,  the  molecxiles  and  atoms  of  which  material  bodies 
are  composed.  In  this  class  we  may  reckon  heat,  latent  and 
specific  heats,  energy  of  gases,  and  perhaps  chemical  energy. 

3d.  All  forms  of  energy  which  have  to  do  with  strains  which 
can  exist  outside  of  ordinary  matter,  i.e.,  every  kind  of  radiant 
energy  and  presumably  electrical  energy. 

These  classes  are  not  absolutely  distinct;  for  example,  we 
do  not  know  the  relation  of  chemical  energy  to  the  third  class, 
nor  of  gravitational  energy  to  any  class,  but  such  a  division 


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ELEMENTARY  PRINCIPLES,  5 

serves  to  keep  clearly  in  our  minds  the  kind  of  actions  to 
which  our  attention  is  to  be  directed. 

It  is  only  within  the  past  few  years  that  we  have  been  able 
with  any  certainty  to  classify  electrical  energy,  and  even  now 
much  remains  to  be  learned.  P'or  a  very  long  while  it  has 
been  known  that  light,  i.e.,  luminous  energy,  must  be  propa- 
gated through  a  medium  quite  distinct  from  ordinary  matter 
and  possessing  certain  remarkable  properties.  It  was  well 
known  that  luminous  energy  is  transferred  through  this 
medium  by  vibratory  or  wave  motion.  Even  the  period  of 
the  vibrations  and  the  lengths  of  the  waves  were  accurately 
measured,  and  from  these  and  similar  measurements  it  has 
been  possible  to  classify  the  mechanical  properties  of  this 
medium,  universally  called  "the  ether,"  until  we  really  know 
more  about  them  than  about  the  properties  of  many  kinds  of 
ordinary  matter  —  a  number  of  the  rare  metals,  for  example. 

The  next  important  step  was  the  discovery,  verified  in  the 
most  thorough  manner,  that  what  had  been  known  as  radiant 
heat,  such  as  we  get  from  the  sun  or  any  very  hot  body,  is 
really  energy  of  the  same  kind  as  light.  That  is,  it  was  found 
to  be  energy  of  wave  motion  of  precisely  the  same  character 
and  in  the  same  medium,  differing  only  in  frequency  and  wave 
length.  It  also  has  turned  but  in  similar  fashion  that  what 
had  been  called  "actinic"  rays,  that  are  active  in  attacking  a 
l)hotographic  plate  and  producing  some  other  kinds  of  chemi- 
cal action,  are  only  light  rays  of  shorter  wave  length  than 
usual,  and  so  ordinarily  invisible  to  the  eye. 

So  much  having  been  ascertained,  it  became  clear  that 
instead  of  three  kinds  of  energy  —  "heat,  light, and  actinism," 
we  were  really  dealing  with  only  one  —  radiant  energy,  \nbrat- 
ing  energy  in  the  ether,  varying  in  effect  as  it  varies  in  fre- 
quency. Speaking  in  an  approximate  way,  such  wave  energy 
has  a  frequency  of  six  hundred  thousand  billion  vibrations  per 
second  and  a  velocity  of  propagation  of  about  a  hundred  and 
eighty-five  thousand  miles  per  second,  so  that  each  wave  is 
not  far  from  one  fifty-thousandth  of  an  inch  long.  These 
dimensions  are  true  of  light  waves;  chemical  action  can  be 
produced  by  waves  of  half  the  length,  while  so-called  heat 
rays  may  be  composed  of  waves  two  or  three  times  as  long  as 


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6  ELECTRIC   TRANSMISSION  OF  POWER. 

those  of  light.  Such  figures  are  startling,  but  they  can  be 
verified  with  an  accuracy  greater  than  that  of  ordinary 
mechanical  measurements. 

We  see  that  this  radiant  energy  is  capable  of  producing 
various  disturbances  perceptible  to  our  senses,  such  as  chemi- 
cal action,  light,  and  heat,  and  that  these  different  effects 
simply  correspond  to  waves  of  energy  having  different  fre- 
quencies and  wave  lengths.  This  being  so,  it  is  not  umiatural 
to  suppose  that  at  still  different  frequencies  other  effects 
might  be  noted.  This  idea  gains  further  probability  from  the 
experimental  fact  that  waves  of  very  different  frequency 
traverse  the  ether  with  precisely  the  same  velocity,  showing  no 
signs  of  slowing  down  or  dying  out,  so  that  there  seems  to  be 
no  natural  limit  to  their  length. 

During  the  past  half  dozen  years  it  has  been  clearly  shown 
that  "radiant  energy"  is  capable  of  producing  profound 
electrical  disturbances,  such  as  violent  oscillations  of  electrical 
energy  in  conducting  bodies,  and  that  these  effects  exist  what- 
ever the  frequency  of  the  ether  waves  concerned.  This  ver3' 
important  fact  was  clearly  foreseen  by  Maxwell  more  than 
twenty  years  ago,  regarding  light,  and  his  prediction  has  been 
thoroughly  verified  through  the  persistent  researches  of  the 
late  Professor  Hertz  and  others. 

This  discovery  is  often  expressed  by  saying  that  radiant 
energy  is  an  electro-magnetic  disturbance,  or  that  light  is  one 
kind  of  electrical  action.  It  is  more  strictly  accurate  to  say 
that  radiant  energy,  just  as  it  produces  chemical  disturbances 
on  the  photographic  plate,  affects  the  eye  as  light,  and  material 
bodies  as  heat,  is  also  capable  of  producing  electrical  effects 
when  transferred  to  the  proper  media.  Most  of  our  experi- 
ments on  its  electrical  effects  have  been  performed  with  waves 
many  thousand  times  longer  than  those  of  light,  but  their  gen- 
eral character  has  proved  to  be  exactly  the  same. 

A  given  substance  may  be  differently  related  to  waves  of 
radiant  energy  of  different  lengths,  but  the  phenomena  are 
still  essentially  the  same.  For  instance,  a  plate  of  hard 
rubber  is  thoroughly  opaque  to  waves  of  a  length  correspond- 
ing to  light,  but  is  quite  transparent  to  those  of  considerably 
greater  length,   such   as   can   produce   thermal   or   electrical 


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ELEMENTARY  PRINCIPLES.  7 

effects.  A  plate  of  alum  will  let  through  light  waves  and  very 
long  waves,  but  will  stop  most  of  those  which  are  efficient  in 
producing  heat.  A  thick  sheet  of  metal  is  quite  opaque  to  all 
known  waves  of  radiant  energy.  Hence  the  fact  noted  long 
ago  by  Maxwell,  that  all  good  conductors  are  opaque  to  light, 
although  the  converse  is  not  true. 

The  substance  of  all  this  is,  that  the  same  sort  of  disturb- 
ance in  the  ether  which  produces  light  is  also  competent  to  set 
up  electrical  actions  in  material  bodies,  and  conversely,  such 
actions  may  and  do  produce  corresponding  disturbances  in  the 
ether,  which  are  thus  transferred  to  other  bodies.  Such  a 
transference  corresponds  to  all  that  we  know  concerning  the 
velocity  with  which  electrical  and  electro-magnetic  disturb- 
ances pass  from  body  to  body.  It  is  equally  certain  that  this 
velocity  totally  transcends  anything  we  could  hope  to  obtain 
from  bodies  having  the  dynamical  properties  of  ordinary 
matter,  while  it  does  fit  exactly  the  dynamical  properties  of 
the  ether. 

We  are  thus  forced  to  the  conclusion  that  when  an  electrical 
current,  as  we  say,  "passes  along"  a  wire,  whatever  a  "cur- 
rent" may  be,  it  is  not  simply  transferred  from  molecule  to 
molecule  in  the  wire  as  soimd  or  heat  would  be,  but  that  there 
is  an  immensely  rapid  transfer  of  energy  in  the  neighboring 
ether  that  reaches  all  points  of  the  wire  almost  simultaneously. 
It  takes  a  measurable  time  for  the  electrical  energy  to  reach 
and  utilize  the  centre  of  the  wire,  although  its  progress  along 
the  surface,  thanks  to  the  free  ether  outside,  is  enormously 
rapid. 

Thus  takes  place  what  is  generally  called  a  "flow  of  elec- 
tricity" along  the  wire.  Looking  at  the  process  more  closely, 
the  nearest  approach  to  flow  is  the  transfer  of  energy  along 
the  wire  by  means  of  stresses  in  the  ether  which  in  turn  set  up 
strains  in  the  matter  along  their  course. 

Whenever  we  cause  in  matter  the  particular  stress  which  we 
call  electromotive  force  for  lack  of  a  more  exact  name,  the 
resulting  strain  is  electrification,  and  if  the  stress  be  applied 
at  one  point  of  a  conducting  body,  the  strain  is  immediately 
transferred  to  other  points  by  the  stresses  and  strains  in  the 
surrounding  ether.     Wherever  this  transference  of  strain  exists 


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8  ELECTRIC  TRANSMISSION  OF  POWER. 

we  have  an  electrical  current,  although  this  name  is  generally 
reserved  for  those  cases  in  which  there  exists  a  perceptible 
transference  of  energy  by  the  means  aforesaid.  If  the  condi- 
tions are  such  that  energy  must  be  steadily  sxipplied  to  keep 
up  the  electromotive  stress,  we  have  such  a  state  of  things  as 
we  find  in  a  closed  circuit  containing  a  battery. 

To  cause  such  a  flow  of  energy  we  must  first  find  means  of 
setting  up  electromotive  stress  capable  of  being  propagated 
through  the  ether.  Now  atoms  and  molecules  are  the  only 
handles  by  which  we  can  get  hold  of  the  ether.  Only  in  so 
far  as  we  can  work  through  them  can  we  do  work  on  the 
ether. 

As  a  matter  of  fact,  we  cannot  do  work  of  any  kind  on  the 
molecules  of  a  body  without  setting  up  electrical  stresses  of 
some  sort.  In  most  cases  of  mechanical  work,  which  in  the 
main  produces  stress  on  the  molecules  only  by  strains  in 
the  mass,  the  energy  appears  mainly  as  heat,  and  is  only  inci- 
dentally electrical,  as  for  instance  the  energy  wasted  in  a 
heated  journal. 

When,  however,  by  any  device  we  do  work  more  directly  on 
the  molecules  of  a  body,  or  on  the  atoms  which  compose  the 
molecules,  we  are  more  than  likely  to  transform  much  of  this 
work  into  electrical  energy.  As  a  rough  example  of  the  two 
kinds  of  action  just  mentioned,  pounding  a  body  heats  it  with- 
out causing  any  considerable  electrification,  while  on  the  other 
hand  rubbing  it  rather  gently,  sets  up  a  considerable  electrifica- 
tion without  heating  it  noticeably. 

In  fact,  for  many  centuries,  friction  was  the  only  known 
method  of  causing  electrification.  Later,  as  is  well  known,  it 
was  discovered  that  certain  sorts  of  chemical  action,  which  has 
to  do  directly  with  interchanges  of  energy  between  molecxiles, 
were  very  potent  in  electrical  eflPects.  With  this  discovery 
came  the  ability  to  deal  with  steady  transfers  of  electrical 
energy  in  considerable  amount  (electric  currents),  instead  of 
the  relatively  slight  and  transitory  effects  previously  known 
(electrification,  "frictional"  electricity). 

To  clear  up  the  real  nature  of  this  difTerence  it  is  well  to 
consider  what  we  mean  by  saying  that  a  body  is  electrified,  or 
has  an  electrical  charge.     In  other  words,  what  is  electrifica- 


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ELEMENTARY  PRINCIPLES.  9 

tion?  Not  very  many  years  ago  this  qxiestion  would  have 
been  answered  by  saying  that  a  quantity  of  a  substance,  posi- 
tive electricity  (or  negative  as  the  case  might  be),  had  been 
communicated  to  the  body  in  question;  that  this  remarkable 
substance  could  reside  only  at  the  surface  of  the  body  and 
was  able  to  produce  in  surrounding  bodies  exactly  an  equal 
quantity  of  negative  electricity;  that  this  '* charge"  of  elec- 
tricity would  repel  another  ^'charge"  of  the  same  substance 
placed  near  it,  or  attract  a  charge  of  its  opposite,  the  other 
substance  called  negative  electricity;  and  mxich  more  to  the 
same  effect.  All  this  was  a  very  convenient  hypothesis  —  it 
explained,  after  a  fashion,  the  common  facts  and  enabled 
investigators  to  discover  many  important  electrical  relations 
and  laws.  But  it  expressed  much  more  than  there  was  any 
reason  to  know.  From  the  standpoint  of  our  modem  doc- 
trines of  energy,  electrification  is  a  very  different  thing. 

Let  an  electromotive  stress  (from  whatever  source)  be 
applied  to  a  body,  a  metallic  sphere  for  example,  long  enough 
to  transfer  to  it  a  finite  amount  of  energy.  This  energy 
appears  as  stresses  and  strains  in  the  ether  everywhere  about 
the  body  under  consideration  and  thence  extends  to  the  mole- 
cules and  atoms  of  neighboring  bodies,  causing  "induced 
charges."  It  is  as  if  one  were  to  fill  a  box  with  jelly,  and  then 
pull  or  push  or  twist  a  rod  embedded  in  its  centre.  The 
result  would  be  strains  in  the  rod,  the  jelly,  and  the  box,  and  in 
a  general  way  the  total  stress  on  the  box  woxild  equal  that  on 
the  rod.  By  proper  means  we  could  detect  the  strain  all 
through  the  substance  of  the  jelly,  but  most  easily  by  its  varia- 
tions from  place  to  place. 

We  do  not  know  exactly  what  sort  of  a  strain  in  oxir  ether 
jelly  is  prodxiced  l>y  electromotive  stress,  but  we  do  know  that 
it  possesses  the  quality  of  endedness,  so  that  the  strains  in 
the  matter  concerned,  i.e.,  in  the  ball  and  surrounding  bodies, 
are  equal  and  opposite. 

In  fact,  the  two  "charges"  are  in  effect  the  two  ends  of  the 
same  strain  in  the  ether.  They  appear  to  us  to  be  real  attri- 
butes of  the  two  opposed  surfaces,  because  at  these  surfaces 
the  dynamical  constants,  such  as  density,  elasticity,  etc.,  of 
the  medium  through  which  the  strain  is  propagated,  change 


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10  ELECTRIC   TRANSMISSION  OF  POWER. 

in  value,  and  differences  in  state  of  strain  there  become  physi- 
cally manifest. 

In  electric  currents  we  have  a  very  different  state  of  things. 
The  energy  supplied  by  the  electromotive  stress,  instead  of 
becoming  potential  as  electrostatic  strain,  and  producing 
"charge,"  does  work  and  is  transformed  into  other  kinds  of 
energy,  thermal  or  chemical,  mechanical  or  luminous. 

When  a  stress  of  whatever  kind  is  applied  to  a  body,  only  a 
limited  amount  of  energy  can  be  transferred  by  it  so  long  as 
the  energy  remains  potential.  Thus,  in  our  box  of  jelly  before 
referred  to,  a  twist  of  given  intensity  applied  to  the  stick,  as 
for  instance  by  a  string  wound  around  it  and  pxilled  by  a  given 
weight,  can  only  transfer  energy  until  the  stresses  produced  in 
the  jelly  come  to  an  equilibrium  with  it.  On  the  other  hand,  if 
the  box  were  filled  with  water  and  the  stick  were  the  axle  of  a 
sort  of  paddle  wheel,  the  very  same  intensity  of  twist  could  go 
on  communicating  energy  to  the  water  as  long  as  one  chose 
to  apply  the  necessary  work. 

This  roughly  expresses  the  difference  between  electric 
charge  and  electric  current,  viewed  from  the  standpoint  of 
energy.  An  electromotive  stress  applied  to  a  wire  charges  it 
and  then  the  transfer  of  energy  ceases.  If  the  same  stress  be 
applied  under  conditions  that  allow  work  to  be  done  by  it, 
energy  will  be  transferred  so  long  as  the  stress  is  kept  up.  In 
an  open  electric  circuit  we  have  a  charge  as  the  result  of  elec- 
tromotive stress.  When  the  circxiit  is  closed,  i.e.,  when  a 
continuous  medium  is  furnished  on  which  work  can  be  done, 
we  have  an  electric  current.  The  amount  of  this  work  and 
the  flow  of  electrical  energy  that  produces  it  depend  on  the 
nature  of  the  circuit.  Certain  substances,  especially  the 
metals,  and  of  metals  notably  copper  and  silver,  permit  a 
ready  continuous  transfer  of  energy  in  and  about  them.  Such 
substances  are  called  good  conductors.  The  real  transfer 
of  energy  takes  place  ultimately  via  the  ether,  but  its  amount 
is  limited  by  the  amount  and  character  of  the  matter  through 
which  work  can  be  done. 

Whenever  the  strains  in  the  ether,  such  as  we  recognize  in 
connection  with  electrical  charge,  shift  through  space  as  when 
a  current  is  flowing,  other  strains  bearing  a  certain  relation 


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ELEMENTARY  PRINCIPLES.  11 

to  the  direction  of  flow  are  made  manifest.  Where  there 
is  a  rapid  and  intense  flow  of  energy,  these  strains  are  very 
great  and  important  compared  with  any  electrostatic  strains 
that  exist  outside  the  conducting  circuit.  In  other  cases  they 
may  be  quite  insignificant.  These  strains  are  electro-magnetic, 
and  with  them  we  have  to  do  almost  exclusively  in  practical 
electrical  engineering.  They  appear  wherever  there  is  a  moving 
electrical  strain,  whether  produced  by  moving  a  charged  body 
or  causing  the  charge  upon  a  body  to  move. 

Both  kinds  of  strains  exist  in  radiant  energy,  as  in  other 
cases  of  flowing  energy.  The  stresses  in  electro-magnetic 
energy  are  at  right  angles  both  to  the  electrostatic  stresses 
and  to  the  direction  of  their  motion  or  flow.     If,  for  example, 


Fi«}.  1.  Fid.  2. 

we  have  a  flow  of  electrical  energy  in  a  straight  wire  (Fig.  1), 
the  electro-magnetic  stresses  are  in  circles  about  it. 

If  A  be  a  wire  in  which  the  flow  of  energy  is  straight  down 
into  the  paper,  the  electro-magnetic  stresses  are  in  circles  in 
the  direction  shown  by  the  arrow  heads.  If  the  wire  be  bent 
into  a  ring  (Fig.  2),  with  the  current  flowing  in  the  direction  of 
the  arrows,  then  the  electro-magnetic  stresses  will  be  (follow- 
ing Fig.  1)  in  such  direction  as  to  pass  downward  through  the 
paper  inside  the  ring. 

These  electro-magnetic  stresses  constitute  what  we  call  a 
magnetic  field  outside  the  wire.  The  intensity  of  this  field  can 
be  increased  by  increasing  the  flow  of  energy  in  the  desired 
region  in  the  systematic  way  suggested  by  Fig.  2.  If,  for 
example,  we  join  a  number  of  rings  like  Fig.  2  into  a  spiral 
coil  shown  in  section  in  Fig.  3,  in  which  the  current  flows 


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12 


ELECTRIC   TRANSMISSION   OF  POWER. 


downward  into  the  paper  in  the  lower  edge  of  the  spiral, 
there  will  be  pro<luced  a  ma^etic  field  in  which  the  stresses 
have  the  direction  shown  by  the  arrows.  Such  a  spiral  consti- 
tutes a  genuine  magnet,  and  if  suspended  so  as  to  be  free 
to  move  would  take  up  a  north  and  south  position  with  it.s 
right-hand  end  toward  the  north.  In  and  about  the  spiral 
there  exists  a  magnetic  "field  of  force,"  which  is  merely  another 
way  of  saying  that  the  ether  there  is  xmder  electro-magnetic 
stress.  Its  condition  of  strain  is  closely  analogous  to  that 
about  an  electrified  body,  and,  as  in  that  case,  there  is  no 


\    V,X \.^J    I 


Fio.  3. 

work  done  on  the  ether  after  the  strains  are  once  established, 
since  the  energy  then  becomes  potential.  While  this  is  being 
accomplished,  work  is  done  just  as  when  a  body  is  charged. 

If,  now,  setting  up  such  an  electro-magnetic  field  requires 
energy  to  be  spent  by  causing  a  current  to  flow  in  the  spiral, 
we  shoxild  naturally  expect  that  if  the  same  field  could  be  set 
up  by  extraneoas  meahs,  energy  would  momentarily  be  spent 
on  the  spiral  in  producing  stresses  and  strains  similar  to  those 
that  set  up  the  original  field.  This  is  found  to  be  so,  the 
process  working  backward  as  well  as  forward. 

If,  for  example,  we  have  two  rings  (Fig.  4),  and  by  sending 
a  current  around  one,  transfer  energy  to  the  medium  outside 
it,  this  energy  will  set  up  an  electromotive  stress  in  the  other 
ring.  The  direction  of  this  stress  is  not  at  once  obvious,  but 
we  can  get  a  very  clear  idea  of  it  by  considering  the  work 
done.  If  current  is  started  in  A  (Fig.  4),  in  the  direction 
shown,  electro-magnetic  stresses  are  produced  in  the  direction 


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ELEMENTARY  PRINCIPLES,  13 

of  the  arrow  C  If  these  are  to  do  work  on  B,  the  electro- 
motive stress  in  the  latter  cannot  have  such  a  direction  as  to 
set  up  on  its  own  account  a  magnetic  field  that  would  assist 
that  of  A,  otherwise  we  could  increase  the  field  indefinitely 
without  added  expenditure  of  energy.  Therefore,  the  electro- 
motive stress  in  B,  and  hence  the  current,  must  be  in  a  direc- 
tion opposing  the  original  current  in  -4,  as  shown  in  the  figure. 
In  like  manner  if  the  current  in  A  be  stopped  and  the  field 
due  to  it  therefore  changes,  there  are  changes  in  the  electro- 
magnetic stresses  about  By  that  again  set  up  an  electromotive 
stress  in  it.  If,  however,  this  change  of  stress  is  to  do  work, 
the  electromotive  stress  in  B  must  be  of  such  direction  as  to 


PlO.  4. 

opf)ose  by  its  field  the  change  in  the  field  of  A  — i,e,,  it  must 
change  its  direction  and  will  now  give  us  a  current  in  the 
same  direction  as  the  original  one  in  A.  All  this  follows  the 
general  law,  that  if  work  is  to  be  done  by  any  stress  it  must 
be  against  some  other  stress.  There  can  be  no  work  without 
resistance. 

In  Fig.  4  we  have  the  fundamental  facts  of  current  induction 
on  which  depend  most  of  our  modem  methods  of  generating 
and  working  with  electrical  energy.  Summed  up  they  amount 
to  saying  that  whenever  there  is  a  change  in  the  electro-mag- 
netic stresses  about  a  conductor,  work  is  done  upon  it,  depend- 
ing in  direction  and  magnitude  on  the  direction  and  magnitude 
of  the  change  in  the  stresses. 

This  is  equally  true  whether  the  stresses  change  in  absolute 
value  or  whether  the  conductor  changes  its  relation  to  them. 
Thus,  in  Fig.  4,  if  A  carries  an  electrical  current  the  result  on 
B  is  the  same  whether  the  field  of  A  changes  through  cessation 
of  the  current,  or  whether  the  same  change  in  the  stresses 


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14  ELECTRIC  TRANSMISSION  OF  POWER. 

about  B  is  produced  by  suddenly  pulling  B  away  from  A.  The 
rate  at  which  work  is  done  depends  on  the  rate  at  which  the 
stresses  are  caused  to  change,  as  might  be  expected.  So  long 
as  the  stresses  are  constant  with  reference  to  the  conductor  in 
which  current  is  to  be  induced,  no  work  can  be  done  xipon  it. 
These  principles  form  the  foundation  of  the  dynamo,  motor, 
alternating  current  transformer,  and  many  other  sorts  of  elec- 
trical apparatus.  Their  details  may  differ  very  widely,  but  we 
can  get  all  the  fundamental  ideas  from  a  consideration  of  Figs. 
3  and  4.  To  define  somewhat  the  specific  idea  of  the  djmamo, 
consider  what  happens  when  a  conducting  wire  is  thrust  into 
a  magnetic  field  such  as  is  produced  by  a  coil,  as  in  Fig.  5. 
As  in  Fig.  3,  let  the  current  in  the  coil  be  flowing  down- 
ward into  the  paper  in  the  lower  half  of  the  figure.  -4  is  a 
wire  perpendicular  to  the  plane  of  the  paper  in  front  of  the 
coil,  its  ends  being  united  at  any  distant  point  that  is  con- 


ri(^  5. 

venient.  Knowing  that  moving  the  wire  into  the  field  will  set 
up  electromotive  stresses  in  it,  we  can  as  before  determine 
their  direction  by  remembering  that  work  must  be  done. 
That  is  (see  Fig.  1),  the  induced  current  will  flow  through  A 
downward  into  the  paper.  In  passing  out  of  the  field,  the  cur- 
rent would  be  xipward. 

We  have  so  far  neglected  the  rest  of  the  circuit.  To  be 
exact,  we  should  consider  it  as  in  Fig.  6.  Following  the  same 
line  of  reasoning  as  in  Fig.  5,  we  see  that  while  the  ring  A  is 
entering  the  magnetic  field  the  current  induced  in  it  must  be 
opposite  to  that  in  the  inducing  coil  (see  Fig.  4).  When  the 
coil  is  leaving  the  field,  however,  this  direction  will  be  reversed. 
Considering  the  coil  A  as  a  whole,  we  see  that  so  long  as  the 
total  field  tending  to  set  up  stresses  in  it  is  increasing,  a  cur- 
rent will  be  induced  opposed   to  that  in  the  inducing  coil. 


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ELEMENTARY  PRINCIPLES. 


15 


While  the  total  field  is  diminishing,  the  induced  current  will 
be  in  the  other  direction.  The  work  that  is  spent  in  moving 
the  coil  A  will  for  the  most  part  reappear  as  electrical  energy 
in  that  coil.  Arrange  the  parts  of  Fig.  6  so  that  the  motion 
of  A  can  be  accomplished  uniformly  and  continuously,  and 


i-tl-*-++rfi-«-t++-l-- 
*■^■Ti-^-^t-♦■t^-t+4^- 

H-rf-f-f-r+-tf"^-H-H 

Ht-i-4-t-t-i-4-»-|-ff-»-f 

ht -•-+■»- t-T'l-fH-f^t 

d  t)  o  o  d  o  D^c^':C 

I 
FlO.  6. 

we  should  have  a  true,  though  rudimentary,  d)aiamo.  Such 
a  structure  could  be  made  by  fixing  A  to  the  end  of  an  arm 
pivoted  at  the  other  end  and  then  revolving  the  arm  so  that 
at  each  revolution  the  coil  A  would  sweep  through  the  field 
of  the  magnetizing  coil  (see  Fig.  7).  The  result  of  this,  as 
we  have  seen,  would  be  on  entering  the  field,  a  current  in 
one  direction,  and  on  leaving,  a  current  in  the  other.  There 
would  thus  be  an  alternating  current  developed  in  the  ring  A, 


Fig,  7. 

If  it  were  cut  at  some  point,  and  wires  led  down  the  arm  and 
to  two  metal  rings  on  the  axis  B,  we  could  obtain,  by  pressing 
brushes  on  these  rings,  an  alternating  current  in  any  outside 
circuit.  To  make  more  of  the  revolution  of  the  arm  useful, 
we  could  arrange  inducing  coils  in  a  circle  about  B.  There 
would  then  be  an  alternation  as  A  passed  each  coil. 

All  these  devices,   however,  would  produce  comparatively 
weak  effects,  because  it  is  difficult  to  produce  powerful  mag- 


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16  ELECTRIC  TRANSMISSION  OF  POWER, 

netic  stresses  in  so  simple  a  way.  There  are  very  few  materials 
in  which  magnetic  stresses  are  easily  set  up  or  propagated. 
Chief  among  these  is  iron,  which  bears  somewhat  the  same  rela- 
tion to  magnetic  actions  that  copper  does  to  electrical  ones. 
By  giving  to  the  coil  in  Fig.  7  a  core  of  soft  iron,  the  electro- 
magnetic effects  obtained  from  it  would  be  greatly  enhanced. 
They  are  comparatively  fee])le  in  air,  and  the  more  iron  we  put 
in  their  path  the  better.  Developing  this  idea,  we  have  in 
Fig.  8  a  much  better  device  for  setting  up  electric  currents. 
Here  the  coil  of  Fig.  7  is  wound  around  an  iron  core,  the  ends 
of  which  are  brought  near  together.     The  arm  of  Fig.  7  is 


also  of  iron  with  enlarged  ends,  and  the  ring  A  is  replaced  by 
a  coil  of  several  turns. 

The  magnetic  stresses  brought  to  bear  on  the  coil  A  are  thus 
made  comparatively  powerful.  Following  out  on  Fig.  8  the 
reasoning  applied  to  Fig.  7,  we  see  that  considerable  electro- 
motive stresses  would  be  set  up  by  the  revolution  of  X,  alter- 
nating in  direction  at  each  half  revolution.  In  fact,  A  is  the 
armature  of  a  simple  alternating  dynamo,  having  two  poles 
N  and  ;S',  so  called  from  their  magnetic  relations  (see  Fig.  3). 

We  have  not  thus  far  considered  the  source  of  the  electro- 
magnetic field  involved.  It  may  be  obtained  as  shown  by 
utilizing  the  electro-magnetic  stresses  set  up  by  a  wire  convey- 
ing electrical  energy,  or  on  a  small  scale  from  permanent  mag- 
nets. The  essential  fact,  however,  is  that  by  forcing  a  wire 
through  a  region  of  electro-magnetic  stress,  electromotive 
stresses  are  set  up  in  that  wire,  the  action  in  every  case  being 
in  such  direction  as  to  compel  us  to  do  work  on  the  wire. 


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ELEMENTARY  PRINCIPLES.  17 

This  work  appears  as  electrical  energy  in  the  circuit  including 
the  moving  wire. 

Now  return  to  Fig.  5  and  consider  the  effect  if  the  wire  A  is 
carrying  a  steady  flow  of  electrical  energy.  It  will  set  up 
electro-magnetic  stresses  about  it  as  already  described.  If 
the  current  be  downward  into  the  paper  in  A,  these  stresses 
will  be  opposed  to  the  stresses  in  the  field.  Inasmuch  as  we 
have  seen  that  in  setting  up  such  a  current,  work  had  to  be 
done  in  forcing  the  wire  into  the  field,  it  follows  that  given 
such  a  current,  there  must  be  between  its  field  and  that  of  the 
coil  a  repulsive  force  which  had  to  be  overcome  by  doing  the 
work  aforesaid.  In  other  words,  there  must  have  been  a 
tendency  to  throw  A  out  of  the  field  of  the  coil.  Just  as 
work  had  to  be  spent  to  produce  electrical  energy  in  ^4,  so 
electrical  energy  will  be  spent  in  keeping  up  the  stresses 
around  A  that  tend  to  drive  it  out  of  the  magnetic  field.  If 
the  current  in  A  were  in  the  other  direction,  the  stresses  in  its 
field  and  that  of  the  coil  would  be  concurrent  instead  of 
oppose<l,  and  their  resultant  would  tend  to  draw  wire  and  coil 
together,  i.e.,  work  would  have  to  be  spent  to  keep  them 
apart.  This  is  the  broad  principle  of  the  electric  motor.  It 
is  sometimes  referred  to  as  simply  a  reversal  of  the  dynamo, 
but  it  really  makes  no  difference  whether  the  structure  in 
which  the  action  jiLst  described  takes  place  is  well  fitted  to 
generate  current  or  not.  (iiven  a  magnetic  field  and  a  wire 
carrying  electrical  energy,  and  there  will  be  a  force  between 
them  depending  in  direction  on  the  directions  of  the  electro- 
magnetic stresses  belonging  to  the  two.  If  either  element  is 
arranged  so  as  to  move  and  still  keep  up  a  similar  relation 
of  these  stresses  we  have  an  electric  motor.  Whether  so 
arranged  as  to  fulfil  this  condition  with  alternating  currents, 
or  in  such  manner  as  to  require  currents  in  one  direction  only, 
the  principle  is  the  same. 

So  far  as  unidirectional  or  "continuous"  currents  are  con- 
cerned they  are  usually  obtained  from  dynamo  electric  machines 
similar  in  principle  to  Fig.  8.  This  machine,  if  the  ends  of 
the  winding  on  the  armature  be  connected  to  two  metal  rings 
insulated  from  each  other,  serves  as  a  source  of  alternating 
currents  which  can  be  taken  off  the  two  rings  by  brushes 


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18  ELECTRIC  TRANSMISSION  OF  POWER, 

pressed  against  them.  If  it  is  necessary  to  obtain  currents  in 
one  direction  only,  this  can  be  readily  done  by  reversing  the 
connection  of  the  outside  circuit  to  the  windings  at  the  same 
moment  that  the  current  reverses  in  them.  The  simplest  way 
of  doing  this  is  by  a  "two  part  commutator,"  such  as  is 
shown  in  diagram  in  Fig.  9.  Here  A  is  the  shaft  surrounded 
by  an  insulating  bushing.  On  this  are  fitted  two  half  rings,  C 
and  C\  of  metal  (the  commutator  segments).  On  these  bear 
brushes  B  and  B\  If  the  ends  of  the  winding  are  connected 
to  C  and  C,  and  the  brushes  are  so  placed  that  they  pass  from 
one  segment  to  the  other  at  the  moment  when  the  current  in 
the  winding  changes  its  direction,  the  direction  of  the  current 


with  respect  to  the  brushes  and  the  outside  circuit  with  which 
they  are  connected  obviously  remains  constant. 

In  the  actual  practice  of  dynamo  building  very  many  refine- 
ments have  to  be  introduced  to  serve  various  purposes,  but  the 
underlying  principle  remains  the  same,  i,e.,  to  set  up  in  a  con- 
ductor electromotive  stresses  by  dragging  it  into  and  out  of 
the  strained  region  of  ether  under  an  electro-magnetic  stress. 

According  as  the  dynamo  is  intended  for  producing  con- 
tinuous or  alternating  currents,  its  structure  is  somewhat 
modified  with  its  particular  use  in  view.  These  modifications 
extend  not  only  to  the  general  arrangement  but  to  the  details 
of  the  winding.  Alternating  dynamos  usually  have  a  more  com- 
plicated magnetic  structure  than  continuous  current  macliines, 
and  are  almost  invariably  separately  excited,  i.e.,  have  their 
magnetizing  current  supplied  from  a  generator  specialized  for 
producing  continuous  current.  The  magnetic  complication  is 
really  only  apparent,  as  it  consists  merely  of  an  increased  nuni- 


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ELEMENTARY  PRINCIPLES.  19 

ber  of  magnet  poles,  due  to  the  desirability  of  obtaining  toler- 
ably rapid  alternations  of  current. 

D)mamos  designed  for  producing  continuous  current  are 
modified  with  the  armature  as  a  starting  point.  The  winding 
is  very  generally  much  more  complicated  than  that  of  an  alter- 
nator, and  the  commutator  that  serves  to  reverse  the  rela- 
tion of  the  windings  to  the  brushes  at  the  proper  moment  is 
correspondingly  elaborate.  The  magnetic  structure  is  usually 
comparatively  simple.  The  whole  design  is  necessarily  sub- 
ordinated to  securing  proper  comnmtation.  Continuous  cur- 
rent dynamos  are  almost  universally  self-excited,  that  is,  the 
current  which  magnetizes  the  field  is  derived  from  the  brushes 
of  the  machine  itself.  Whatever  the  character  of  the  machine 
the  electromotive  force  generated  in  it  increases  with  the  inten- 
sity of  the  magnetic  field  (that  is,  with  the  magnitude  of  the 
electro-magnetic  strains  which  affect  the  armature  conductors), 
with  the  speed  (that  is,  with  the  rate  of  change  of  electro- 
magnetic stress  about  these  moving  conductors),  and  with 
the  number  of  turns  of  wire  of  which  the  electromotive  forces 
are  added.  The  capacity  of  the  machine  for  furnishing  elec- 
trical energy  varies  directly  with  the  electromotive  force  and 
with  the  capacity  of  the  armature  conductors  for  transmitting 
the  energy  without  becoming  overheated.  Practically  all  the 
energy  lost  in  a  dynamo  appears  in  the  form  of  heat,  which 
must  be  limited  to  an  amount  which  will  not  cause  an  undue 
rise  of  temperature. 

It  is  not  the  purpose  of  this  chapter  to  deal  with  the  prac- 
tical details  of  dynamo  design  and  construction.  For  these, 
the  reader  should  consult  special  treatises  on  the  subject, 
which  consider  it  with  a  fulness  which  would  here  be  quite 
out  of  place.  Special  machines,  however,  will  be  briefly  dis- 
cussed in  their  proper  places  and  in  relation  to  the  work  they 
have  to  do. 

Having  now  considered  the  principles  which  underlie  the 
transformation  of  mechanical  into  electrical  energy,  we  may 
profitably  take  up  the  fundamental  facts  in  regard  to  the 
measurement  of  that  form  of  energy  and  the  units  in  which  it 
and  its  most  important  factors  are  reckoned. 

All  electrical  quantities  are  measured  directly  or  indirectly 


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20  ELECTRIC  TRANSMISSION  OF  POWER. 

in  terms  of  the  dynamical  units  foimded  upon  the  imits  of 
length,  mass,  and  time.  These  derived  dynamical  units  can 
serve  alike  for  the  measurement  of  all  forms  of  energy,  so 
that  all  have  a  common  ground  on  which  to  stand.  As  the 
electrical  units  are  derived  directly  from  the  same  units  that 
serve  to  measure  ordinary  mechanical  effects,  electrical  and  me- 
chanical energies  are  nuitually  related  in  a  perfectly  definite  way. 

A  natural  starting  point  in  the  derivation  of  a  working 
system  of  electrical  units  may  be  found  in  electro-magnetic 
stress,  such  as  is  developed  about  an  electrical  circuit  or  a 
permanent  magnet.  To  begin  with,  the  mechanical  units  that 
may  serve  to  measure  any  form  of  energy  are  derived  from 
those  of  length,  mass,  and  time.  These  latter  are  almost  uni- 
versally taken  as  the  centimetre,  gramme,  and  second,  the 
'*  C.  Ci.  S. "  system.  Starting  from  these  the  unit  of  force  is  that 
which  acting  for  one  second  on  a  mass  of  one  gramme  can 
change  its  velocity  by  one  centimetre  per  second.  This  unit  is 
called  the  dyne,  and  as  a  magnetic  stress  it  is  equivalent  to  a 
push  of  about  ?4  5*<jiyxF  ^f  a  pound's  weight  on  a  similar  "imit 
pole"  one  centimetre  distant.  This  unit  is  inconveniently  small 
for  practical  use,  and  before  long  some  nuiltiple  of  it  is  likely 
to  be  given  a  special  name  and  used  for  practical  reference.  In 
fact,  one  megadyne  (i.e.,  1,()()(),(K)()  dynes)  is  very  nearly  equiva- 
lent to  the  weight  of  a  kilogramme.  Magnetic  measiu'e- 
ments  may  thus  be  made  by  direct  reference  to  the  dyne  and 
centimetre,  since  the  unit  ]>()le  is  that  which  repels  a  similar 
pole  1  centimetre  distant,  with  a  force  of  1  dyne. 

Referring  now  to  what  has  been  said  about  the  causes  which 
vary  the  electromotive  force  produced  in  a  dynamo,  we  fall  at 
once  into  the  definition  of  the  unit  electromotive  force,  which 
is  that  produced  when  field,  velocity,  and  length  of  wire  imder 
inducticm  are  all  of  unit  value.  The  unit  electromotive  force 
is,  then,  that  which  is  generated  in  one  centimetre  of  wire 
moving  one  centimetre  per  second,  perpendicular  to  its  own 
length,  straight  across  unit  field,  which  is  that  existing  one 
centimetre  from  unit  pole  as  indicated  above.  This  unit,  too, 
is  inconveniently  small,  so  that  one  hundred  miUion  times  this 
(|uantity  is  taken  for  the  practical  unit  of  electromotive  force 
and  called  the  volt. 


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ELEMENTARY  PRIXCIPLES.  21 

The  unit  electrical  current  is  that  which  flowing  through  one 
centimetre  length  of  wire  will  create  unit  field  at  any  point 
equidistant  from  all  parts  of  the  wire  (as  when  the  wire  is 
bent  to  a  curve  of  1  centimetre  radius).  One-tenth  of  this  cur- 
rent is  taken  as  the  working  unit  and  called  the  ampere. 

The  unit  electrical  resistance  (one  ohm)  is  that  through 
which  an  electromotive  force  of  one  volt  will  force  a  current  of 
one  ampere. 

The  C.  G.  S.  unit  of  work  is  that  due  to  unit  force  acting 
through  unit  distance;  that  is,  one  dyne  acting  through  one 
centimetre.  As  this  is  too  small  to  be  generally  convenient, 
ten  million  times  this  amoimt  is  taken  as  the  working  unit 
(called  the  joule).  This  is  a  little  less  than  three-quarters  of  a 
foot-pound  (exactly  .7373).  The  unit  rate  of  doing  work  is 
one  joule  per  second.  This  unit  rate  is  called  the  watt,  and 
translating  this  into  English  measure,  one  watt  ecjuals  tA 
horse- power. 

Although  the  watt  is  often  spoken  of  as  an  electrical  unit,  it 
belongs  no  more  to  electrical  than  to  any  other  form  of  energy. 
It  only  remains  to  show  the  relation  of  the  watt  to  the  more 
strictly  electrical  units  just  mentioned.  Recurring  to  our 
definition  of  the  volt,  let  us  suppose  that  the  resistance  of  the 
circuit  of  which  the  moving  wire  is  a  part  is  such  that  unit 
electromotive  force  produces  unit  current  in  it.  The  stress 
between  the  field  of  the  moving  wire  and  the  other  unit  field 
in  which  it  moves  is  then  one  dyne  at  unit  distance.  In  main- 
taining this  for  one  second  at  the  given  rate  of  moving  (1  cm. 
per  second)  the  work  done  is,  as  above,  one  C.  G.  S.  unit.  At 
this  rate,  if  the  E.  M.  F.  were  1  volt  and  the  current  1  ampere, 
the  work  would  be  one  joule  and  the  rate  of  doing  work  one 
watt.  If  either  E.  M.  F.  or  current  were  changed,  the  work 
would  be  proportionally  changed.  So,  the  number  of  volts 
multiplied  by  the  number  of  amperes  is  numerically  equal  to 
the  watts,  i.e.,  we  have  obtained  the  dynamical  equivalent 
of  the  two  factors  that  make  up  electrical  energy  as  ordinarily 
reckoned.  So  the  output  of  any  dynamo  in  watts  is  deter- 
mined by  the  volt-amperes  produced,  and  we  see  the  reason 
of  the  ordinary  statement  that  746  volt-amperes  make  one 
horse-power.     This  is  always  true  whether  the  output  is  steady 


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22  ELECTRIC   TRANSMISSION  OF  POWER. 

or  variable,  so  long  as  we  give  to  the  product  of  volts  and 
amperes  their  true  concurrent  values. 

What  few  other  electrical  units  appear  in  practical  work  will 
be  referred  to  in  their  proper  places. 

It  has  been  the  purpose  of  this  chapter,  not  so  much  to  set 
forth  the  ordinary  elements  of  electrical  study,  as  to  present 
these  elements  as  viewed  from  the  standpoint  of  energy.  The 
author  has  purposely  avoided  the  conception  of  electricity  as 
a  material  something,  to  lay  the  greater  emphasis  on  the 
paramount  importance  of  electrical  energy.  The  present 
recrudescence  of  a  material  theory  of  electrical  charge  in  no 
way  affects  the  validity  of  the  principles  here  laid  down,  since 
it  deals  merely  with  a  possible  mechanism  behind  the  stresses 
and  strains  which  are  experimentally  apparent. 


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CHAPTER  II. 

GENERAL   CONDITIONS   OF    POWER   TRANSMISSION. 

The  growth  of  human  industry  depends  on  nothing  more 
than  upon  the  possession  of  cheap  and  convenient  power. 
Labor  is  by  far  the  largest  factor  in  the  cost  of  many  manu- 
factured articles,  and  in  so  far  as  motive  power  is  cheap  and 
easy  of  application  it  tends  to  displace  the  strength  of  human 
hands  in  all  manufacturing  processes,  and  so  to  reduce  the 
labor  cost  and  to  set  free  that  labor  for  other  and  less  purely 
machine-like  purposes. 

Therefore  industrial  operations  have  steadily  gravitated 
toward  regions  where  power  is  easily  procured,  often  at  the 
sacrifice  of  certain  other  advantages.  This  is  in  no  wise 
better  shown  than  by  the  growth  of  cities  around  easily  avail- 
able water-powers,  even  in  regions  where  both  raw  material 
and  finished  product  became  subject  to  considerable  cost  of 
transportation.  With  the  introduction  of  the  steam  engine 
came  a  corresponding  tendency  to  gather  factories  about 
regions  of  cheap  fuel.  These  localities,  like  those  in  which 
water-power  is  plentiful,  seldom  coincide  with  centres  of  cheap 
material  and  transportation,  so  that  it  has  generally  been 
desirable  to  strike  an  average  condition  of  maximum  economy 
by  transporting  the  necessary  power,  stored  in  the  form  of 
fuel,  to  some  advantageous  point. 

Experience  has  shown,  however,  that,  while  the  hauling  of 
coal  is  a  simple  and  comparatively  cheap  expedient,  fuel 
utilized  for  running  heat  engines  is  in  very  many  cases  so 
much  more  expensive  than  hydraulic  power  as  to  be  quite  out 
of  competition  in  cases  where  the  latter  can  be  transmitted, 
with  a  reasonable  degree  of  economy,  to  places  that  are  favor- 
able for  its  utilization.  And  in  general  it  is  found  that  there 
is  a  wide  field  for  the  transmission  of  power  obtained  from  a 
given  source,  in  competition  with  power  from  some  other 
source  utilized  in  situ, 

23 


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24  ELECTRIC   TRANSMISSION  OF  POWER. 

The  sources  of  energy  on  which  we  may  draw  for  mechanical 
power  to  be  employed  on  the  spot  or  transmitted  elsewhere 
are  very  diversified,  although  few  of  them  are  to-day  utilized 
in  any  considerable  amount.  Taking  them  in  the  order  of 
their  present  importance  we  arrive  at  something  like  the  fol- 
lowing classification: 
I.  Fuel, 
II.   Water-power. 

III.  Wind. 

IV.  Solar  radiation. 

V.   Tidal  and  wave  energy. 

VI.   Internal  energy  of  the  earth. 

Of  these  only  the  first  two  play  any  important  part  in  our 
industrial  economy.  The  third  is  employed  in  a  very  small 
and  spasmodic  way,  the  fourth  and  fifth  although  enormous  in 
amount  are  almost  untouched,  while  the  last  is  not  at  present 
used  at  all,  owing  to  inherent  difficulties. 

I.  The  world's  supply  of  fuel  is  almost  too  great  for  intel- 
ligible description.  Aside  from  a  widely  distributed  and 
steadily  renewed  supply  of  wood,  the  extent  and  capacity  of 
available  coalfields  give  promise  that  for  a  very  long  time  to 
come  fuel  will  be  the  chief  source  of  energy.  Coal  is  found 
in  nearly  every  country,  and  in  most  quite  plentifully,  while 
exploration  both  in  old  fields  and  in  new,  is  constantly  bringing 
to  light  fresh  supplies.  Many  computations  concerning  the 
probable  duration  of  the  coal  supply  have  been  made,  but  they 
are  generally  unreliable  owing  to  the  great  probability  that 
only  a  very  small  proportion  of  the  available  coal  is  as  yet 
known  to  mankind.  Certaui  it  is  that  there  is  unlikely  to  be 
a  marked  scarcity  of  fuel  for  several  centuries  to  come,  even  at 
the  present  rate  of  increase  in  it-s  consumption.  Still,  it  is 
altogether  probable  that  it  may  become  considerably  clearer 
than  at  present  within  perhaps  the  present  century,  owing  to 
the  increased  difficulty  of  working  the  older  mines  and  the 
comparative  inaccessibihty  of  new  ones. 

Besides  coal  we  have  petroleum  and  natural  gas  in  unknown 
but  surely  very  great  quantities,  since  the  distribution  of  both 
is  far  wider  than  has  generally  been  supposed.  At  present  the 
cost  of  these  as  fuel  does  not  differ  widely  from  that  of  coal, 


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GENERAL  COXDiriOXS  OF  POWER   TRAXSMISSIOX,    25 

but  appearances  indicate  that  they  are  likely  to  be  sooner 
exhausted. 

Every  improvement  that  is  made  in  the  generation  of  power 
by  st^am  and  its  subsequent  distribution,  helps  to  economize 
the  fuel  supply  and  stave  off  the  already  distant  day  when  fuel 
shall  be  scarce.  The  work  of  the  past  half  century  has  by  direct 
improvement  in  steam  practice,  nearly  if  not  quite  doubled 
the  energy  available  per  ton  of  fuel.  Beyond  this  much  has 
been  done  along  collateral  lines.  Particularly,  explosive  vapor 
engines  have  been  developed  to  a  point  at  which  they  are  for 
small  powers  decidedly  more  economical  than  steam  engines. 
Gas  engines  of  moderate  size,  5  to  25  HP,  are  readily  ob- 
tained of  such  excellence  as  to  give  a  brake-horse-power  hour 
on  an  expenditure  of  little,  if  any,  more  than  20  cu.  ft.  of  ordi- 
nary gas,  reducing  the  cost  per  HPH  to  below  that  of  power 
from  a  steam  engine  of  similar  size.  Engines  using  an  explo- 
sive mixture  of  air  and  petroleum  vapor  are  at  least  equally 
economical,  in  fact  more  so  unless  the  comparison  be  made 
with  very  cheap  gas. 

These  explosive  engines  have  nearly  double  the  net  efficiency 
of  steam  engines  as  converters  of  thermal  energy  into  mechani- 
cal power,  and  are  capable  of  giving  under  favorable  circum- 
stances 1  HPH  on  the  thermal  e(iuivalent  of  less  than  1  pound 
of  coal. 

IL  Water-power  derived  from  streams  is  not  distributed  with 
the  same  lavish  impartiality  as  fuel,  but  nevertheless  exists  in 
many  regions  in  sufficient  amount  to  be  of  the  greatest  impor- 
tance in  industrial  operations.  Available  streams  exist  around 
almost  every  mountain  range  and  are  capable  of  furnishing  an 
amount  of  power  that  is  seldom  realized.  In  the  United  States 
the  total  horse-power  of  the  improved  water-power  is  approxi- 
mately 1,500,000.  New  England  is  especially  rich  in  this  re- 
spect, as  is,  too,  the  entire  region  bordering  on  the  Appalachian 
range.  The  Rocky  Mountains  are  less  favored,  the  available 
water  being  rather  small  in  amount,  on  account  of  the  smaller 
rainfall  and  the  severe  cold  of  the  winters. 

The  Pacific  slope  is  rather  better  off,  and  the  high  price  of  coal 
operates  to  hasten  the  development  of  every  practicable  power. 
All  over  the  country  are  scattered  small  water-powers,  and  one 


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26  ELECTRIC  TRANSMISSION  OF  POWER, 

of  the  interesting  results  of  the  growth  of  electrical  power 
transmission  has  been  to  bring  to  light  half  forgotten  falls,  even 
in  familiar  streams.  Abroad,  Switzerland  is  rich  in  powers  of 
moderate  size,  as  is  the  entire  Alpine  region,  while  a  few  years 
of  experience  in  electrical  transmission  will  probably  cause  the 
discovery  or  utilization  of  many  water-powers  that  have  hardly 
been  considered,  even  in  highly  developed  countries.  Of  the 
world's  total  water-power  supply  we  know  little  more  than 
of  its  coal  supply,  but  it  is  quite  certain,  now  that  transmission 
of  power  over  very  considerable  distances  is  practicable,  that 
the  employment  of  the  one  will  every  year  lessen  the  relative 
inroads  upon  the  other.  And  this  is  in  spite  of  the  fact  that 
water  is  by  no  means  always  cheaper  than  steam  as  a  motive 
agent. 

III.  Wind  as  a  prime  mover  has  been  employed  on  a  rather 
small  scale  from  the  very  earliest  times.  Were  it  not  for  the 
extreme  irregularity  of  the  power  supplied  by  it  in  most  places, 
the  windmill  would  be  t()-<lay  a  very  important  factor  in  the 
problem  of  cheap  power.  Unhappily,  winds  in  the  same  place 
vary  most  erratically,  from  the  merest  breeze  to  a  hurricane 
sweeping  along  at  the  rate  of  5()  to  75  miles  an  hour.  As  all 
strengths  of  wind  within  very  wide  limits  must  be  utilized  by  the 
same  apparatus  running  at  all  sorts  of  speeds,  it  is  no  easy 
matter  to  employ  it  for  most  sorts  of  work.  It  seems  especially 
unfitted  for  electrical  work,  and  yet  several  small  private  plants 
have  obtained  good  results  from  windmills  used  in  connection 
with  storage  batteries. 

In  ordinary  winds  the  great  size  of  the  wheel  necessary  for  a 
moderate  power  militates  against  any  very  extensive  use.  For 
example,  with  a  good  breeze  of  10  miles  per  hour  a  wheel  about 
twenty-five  feet  in  diameter  is  needed  to  produce  steadily  a 
single  effective  horse-power,  and  the  rate  of  rotation,  about  30 
revolutions  per  minute,  is  so  low  as  to  be  inconvenient  for  many 
purposes.  Hence  windmills  are  generally  used  for  very  small 
work  which  can  be  done  at  variable  speed,  such  as  pumping, 
grinding,  and  the  like,  for  which  they  are  unexcelled  in  cheap- 
ness and  convenience.  For  large  work  we  can  hardly  count 
much  on  wind-power,  in  spite  of  ingenious  speculations  to  the 
contrary,  and  as  a  source  of  power  for  general  distribution  it 


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GENERAL  CONDITIONS  OF  POWER   TRANSMISSION.    27 

is  out  of  the  question,  for  such  as  it  is  we  have  it  already  dis- 
tributed. It  must  ratlier  be  regarded  as  a  local  competitor  of 
distributed  power,  and  even  so  only  in  a  small  and  limited  field. 

IV.  Aside  from  being  in  a  general  way  the  ultimate  source 
of  nearly  all  terrestrial  energy,  the  sun  steadily  furnishes  an 
amount  of  radiant  energy,  which  if  converted  into  mechanical 
power  would  more  than  supply  all  possible  human  needs.  Its 
full  value  is  the  equivalent  of  no  less  than  ten  thousand  horse- 
power per  acre  of  surface  exposed  to  the  perpendicular  rays  of 
the  sun. 

This  prodigious  amount  is  reduced  by  perhaps  one-third 
through  atmospheric  absorption  before  it  reaches  the  sea  level, 
and  in  cloudy  weather  by  a  very  much  larger  amount.  Never- 
theless, with  clear  sunlight  the  amount  of  energy  practically 
available,  after  making  all  allowances  for  increased  absorption 
when  th6  sun  is  low,  and  for  the  hours  of  darkness  in  any  given 
place,  is  very  great.  If  we  suppose  the  radiant  energy  to  be 
received  cm  concave  mirrors  kept  turned  toward  the  sun  and 
arranged  so  as  to  utilize  the  heat  in  the  boiler  of  a  steam  or 
vapor  engine,  the  average  result  after  making  all  allowances  for 
losses  would  be  one  mechanical  horse-power  for  each  100  square 
feet  of  mirror-aperture,  available  about  ten  hours  per  day. 

Very  important  pioneer  work  was  done  on  solar  engines 
by  John  Ericsson  and  by  M.  Mouchot  more  than  a  quarter 
century  ago,  but  it  is  only  within  the  past  few  years  that  the 
solar  engine  has  approached  really  commercial  form.  At  the 
present  time  solar  heating  apparatus  is  being  regularly  pro- 
duced although  on  a  rather  small  scale,  and  gives  good  economic 
results.  The  solar  motor  is  essentially  a  steam  engine  supplied 
with  steam  by  a  boiler  placed  in  the  focus  of  a  concave  mirror. 
This  is  shaped  like  an  open  umbrella  with  its  handle  pointed 
toward  the  sun.  The  umbrella  is  carried  on  a  polar  axis  at 
right  angles  to  the  handle  and  pointing  toward  the  celestial 
pole.  The  actual  mirror,  is  segmental,  built  upon  a  steel 
frame,  of  rectangles  of  plane  thin  glass  silvered  on  the  back. 
Each  segment  is  about  six  inches  wdde  and  two  feet  long,  sup- 
ported by  cushioned  clamps  at  the  comers,  and  the  whole  are 
arranged  to  focus  the  sun's  rays  on  a  cylindrically  disposed 
blackened   boiler  formed  of  copper  tube.     The  structure  is 


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28  ELECTRIC   TRANSMISSION  OF  POWER. 

supported  on  a  polar  axis  about  which  it  is  moved  automatically 
by  steps  every  few  minutes,  remaining  locked  in  the  intervals 
to  avoid  needless  strain  on  the  clockwork.  There  is  also  a 
motion  in  declination  to  take  care  of  the  apparent  motion  of 
the  sun,  adjusted  by  hand  every  day  or  two  as  becomes  neces- 
sary. The  engine  is  generally  a  rather  highly  organized  one, 
worked  condensing  and  with  superheated  steam  at  a  pressure 
of  200  lbs.  per  square  inch  or  more.  The  net  result  is  one  brake 
HP  for  each  100  square  feet  of  mirror  surface.  The  mirror 
structure  becomes  rather  unwieldy  when  of  dimensions  great 
enough  to  supply  an  engine  of  more  than  15  HP,  so  that  for 
greater  powers  several  mirrors  with  their  boilers  should  be 
coupled  together.  The  initial  cost  of  each  equipment  is  high, 
say  $250  per  horse-power,  but  the  fuel  cost  is  nil  and  the 
attendance  required  very  little,  so  that  even  now  there  are 
localities  where  its  use  is  economical.  The  full  power  is 
available  about  eight  hours  per  day,  and  there  is  upon  the  earth's 
surface  a  vast,  irregular  equatorial  belt  in  which  such  solar 
engines  can  be  successfully  used  for  irrigation  and  other  pur- 
poses. The  power  is  steady,  and  reliable  during  the  hours  of 
sunshine,  and  gives  constant  speed  like  any  other  steam  engine. 
It  is  worth  mentioning  that  general  heating  and  cooking  appa- 
ratus on  the  same  plan  is  entirely  practicable  in  regions  of 
scant  fuel  and  high  sun,  and  has  been  tried  successfully. 

V.  Of  tidal  energy  but  little  use  has  yet  been  made.  Here 
and  there,  both  here  and  abroad,  are  small  tidemjUs,  feebly  sug- 
gesting the  enormous  store  of  tidal  power  as  yet  unutilized. 
The  intermittent  character  of  tidal  currents  and  the  small 
extent  of  the  rise  and  fall  generally  available,  make  the  practical 
part  of  the  problem  somewhat  difficult.  The  easiest  way  of 
harnessing  the  tides  is  to  let  the  rising  water  store  itself  in 
artificial  reservoirs,  or  natural  ones  artificially  improved,  and 
then  during  the  ebb  to  use  it  with  water-wheels.  But  usually 
the  head  is  so  small  that  for  any  considerable  power  stored  the 
area  of  reservoir  must  be  very  large,  and  the  wheels  must  be 
of  great  size  in  order  to  make  the  stored  water  do  its  work 
before  the  rising  tide  checks  further  operations.  The  average 
tide  is  seldom  more  than  10  to  12  feet  along  our  coast,  and  of 
this  hardly  more  than  half  could  be  utilized  to  give  even  a  few 


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GENERAL  CONDITIONS  OF  POWER  TRANSMISSION.    29 

hours  of  daily  service.  At  6  feet  available  head  about  100 
cubic  feet  of  water  must  be  stored  for  each  horse-power-minute, 
even  with  the  best  modem  turbines.  Hence  for  say  1,000 
HP  available  for  5  hours  there  must  be  impounded  30,000,000 
cubic  feet  of  water,  making  a  pond  6  feet  deep  and  almost 
120  acres  in  extent. 

Tidal  operations  are  therefore  likely  to  be  restricted  to  a 
few  favored  localities  where  through  special  configuration  of 
the  ground  natural  reservoirs  can  be  found,  and  where  the  rise 
of  the  tide  is  several  times  the  figure  nameil.  In  rare  cases,  by 
the  use  of  more  than  one  reservoir  and  outlet,  work  may  be 
made  nearly  or  quite  continuous.  Still,  with  all  these  difficul- 
ties the  possibilities  of  tidal  power  are  enormous  in  certain 
cases.  Take  for  example  the  Bay  of  Fundy  with  its  40  feet  of 
normal  tidal  rise.  If  half  this  head  can  be  used  in  practice  30 
cubic  feet  will  be  required  per  horse-power-minute,  and  a  single 
square  mile  of  reservoir  capacity  gained  by  damming  an  estu- 
ary or  cutting  into  a  favorable  location  on  shore  will  yield 
62,000  horse-power  ten  hours  per  day  in  two  five-hour  intervals. 
Generally  speaking,  economic  conditions  are  not  favorable  for 
such  an  employment  of  the  tides,  but  in  some  localities  a 
peculiarly  fortunate  contour  of  the  shore  coupled  with  high 
local  cost  of  fuel  may  render  it  easy  and  profitable  to  press  the 
tides  into  service.  The  author  has  had  cxH^asion  to  investigate 
a  few  cases  of  this  kind  in  which  the  commercial  outlook  was 
good.  The  main  difficulties  in  utilizing  the  tides  are  two:  first, 
the  very  variable  head;  and,  second,  the  short  daily  periods  in 
which  the  outflow  can  be  advantageously  used.  Moreover, 
these  periods  shift  just  as  the  times  of  high  tide  shift,  by  a 
little  less  than  an  hour  per  day,  so  that  if  the  power  were  used 
directly  it  would  often  be  available  only  at  very  inconvenient 
times. 

To  work  the  tides  on  a  really  commercial  scale,  therefore, 
some  system  of  storing  power  is  absolutely  necessary.  And 
since  one  would  have  to  deal  with  very  large  amounts  of 
power,  much  of  the  time  the  entire  output  of  the  plant,  the 
storage  must  be  fairly  cheap  and  efficient.  For  work  on  the 
scale  contemplated,  it  is  probable  that  the  storage  battery  is 
the   most   available   method.       Used   in   very  large   units   in 


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30  ELECTRIC  TRANSMISSION  OF  POWER, 

a  colossal  plant,  most  of  the  serious  objections  to  the 
storage  battery  are  in  great  measure  obviated,  since  attend- 
ance and  repairs  can  be  part  of  the  duties  of  a  regular  main- 
tenance department,  inspecting,  testing,  and  repairing  damaged 
cells,  casting  and  filling  new  plates,  and  keeping  the  plant  in 
first-class  working  condition  all  the  time. 

The  cost  of  battery  tvould  be,  of  course,  a  serious  matter,  but 
not  prohibitive,  and  its  efficiency  could  probably  be  kept  as 
high  as  80  per  cent.  The  best  idea  of  the  economic  side  of 
the  case  can  be  gained  by  investigating  a  hypothetical  case  of 
tidal  storage,  based,  for  convenience,  on  the  square  mile 
of  reservoir  just  mentioned.  To  simplify  the  case  we  will 
assume  use  of  the  power  locally,  so  as  not  to  complicate  the 
situation  by  the  details  of  a  long-distance  transmission.  We 
will  take  the  generators,  which  can  be  worked  at  steady  full 
load,  at  94  per  cent  efficiency.  Then  the  efficiency  to  the  dis- 
tributing lines  would  be 

.94  X  .80  =  .752. 

At  this  rate,  the  62,000  HP  available  would  give  substan- 
tially 35,0(K)  KW;  i.e.,  350,000  KW-hours  daily.  Storage 
capacity  would  have  to  be  provided  for  this  whole  amount 
in  a  gigantic  battery,  weighing  about  18,0(K)  tons  and  cost- 
ing in  the  neighborhood  of  three  million  dollars.  To  this, 
of  course,  the  cost  of  the  electrical  and  hydraulic  machinery 
must  be  added,  and  beyond  this  must  be  reckoned  the  really 
very  uncertain  cost  of  the  reservoir  and  hydraulic  work.  In 
spite  of  all  this,  an  assured  market  for  the  output  would  lead 
to  economic  success  under  conditions  quite  possible  to  be 
realized.  If  extensive  transmission  had  to  accompany  the 
enterprise  there  would  be  still  further  loss  of  efficiency,  so 
that  the  final  figure  would  not  exceed  60  per  cent,  which 
would  reduce  the  salable  power  to  about  27,(X)0  KW.  Evi- 
dently this  would  have  to  command  a  very  good  price,  to  carry 
the  burden  of  the  heavy  investment,  which  would  probably 
rise  to  between  $10,000,000  and  $15,000,000.  The  cost  of 
such  an  enterprise  is  so  formidable  that  it  is  practically  out  of 
the  question,  unless  it  can  reach  a  market  for  j)ower  in  which 
a   ver^'  high   price  is  admissible.     When   fuel   begins   to   get 


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GENERAL  CONDITIONS  OF  POWER  TRANSMISSION,    31 

scarce  it  will  be  profitable  to  utilize  the  tides  on  a  large  scale; 
until  then,  their  use  will  be  confined  to  isolated  cases  in  which 
local  causes  lead  to  high  cost  of  other  power  and  tidal  storage 
is  unusually  cheap. 

All  these  considerations  apply  with  similar  force  to  wave 
motors,  which  have  been  often  suggested,  and  now  and  then 
used,  as  sources  of  power.  The  energy  of  the  waves  is  very 
great,  as  the  havoc  wrought  by  storms  bears  witnass;  but  it  is 
most  irregular  in  amount,  and  requires  very  large  apparatus 
for  its  utilization.  What  is  worse,  the  power  is  intermittent, 
so  that  to  be  of  any  material  advantage  it  must  be  brought  to 
a  steady  output  by  means  of  storage  of  energy  in  some  form. 
The  periodicity  of  wave  motion  is  so  low,  roughly  about  6  to 
10  crests  per  minute,  that  flywheels  and  the  like  are  of  little  use, 
and  storage  is  practically  reduced  to  a  question  of  compressing 
air  or  pumping  water.  Even  if  some  such  wasteful  intermedi- 
ary were  not  necessary,  and  one  could  work  directly  by  means 
of  floats  or  their  equivalent,  a  float  would  have  to  have  a  dis- 
placement of  at  least  one  ton  per  horse-power,  even  if  work- 
ing in  a  pretty  heavy  sea,  and  under  ordinary  circumstances 
several  times  that  amount  of  displacement.  At  best,  wave 
motors  are  cumbersome,  and  give  small  promise  of  economic 
development  while  other  sources  of  energy  are  available. 

VI.  Of  the  earth's  internal  heat  energy  there  is  little  to  be 
said.  It  is  quite  unused  save  as  an  occasional  source  of  hot 
water,  and  except  in  a  very  few  cases  could  not  be  employed  at 
all,  much  less  to  any  advantage.  Immense  as  is  its  aggregate 
araoimt,  it  is,  save  at  isolated  points,  so  far  separated  from  the 
earth's  surface  as  to  be  very  difficult  to  get  at.  Hot  springs, 
very  deep  artesian  wells,  and  some  volcanic  regions,  furnish  the 
only  feasible  sources  of  terrestrial  heat  energy,  so  that  the 
whole  matter  is  only  of  theoretical  interest. 

We  see  that  at  present  only  two  sources  of  energy,  viz., 
fuel  and  water-power,  are  worthy  of  serious  consideration  in 
connection  with  the  general  problem  of  the  transmission  and 
distribution  of  power.  The  other  sources  enumerated  are 
either  very  irregular,  uncertain  in  amount,  or  so  difficult  of 
utilization  as  to  remove  them  at  once  from  the  sphere  of  prac- 
tical work. 


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32  ELECTRIC  TRANSMISSION  OF  POWER. 

Granted,  then,  that  fuel  and  water-power  are  and  are  likely 
long  to  remain  the  dominant  sources  of  energy,  let  us  look  more 
closely  into  their  possibilities.  From  each  energy  can  be 
readily  transmitted  and  distributed  by  any  suitable  means; 
each,  in  fact,  can  be  transferred  bodily  to  a  distant  scene  of 
action  without  any  transformation  from  its  own  proper  form. 
In  fact,  for  certain  purposes  and  under  certain  conditions  such 
is  the  very  best  method.  Fuel  for  ordinary  heating  and  water 
for  such  uses  as  hydraulic  mining  can  be  taken  as  cases  in 
point.  In  a  more  general  way,  both  fuel  and  water  for  the 
development  of  mechanical  power  may  often  profitably  be 
transferred  from  place  to  place. 

The  conditions  of  economy  in  the  transmission  of  fuel  as 
such  are  comparatively  easy  to  examine  and  define.  Coal 
may  be  produced  at  the  mine  for  a  certain  quite  definite 
cost  per  ton.  It  can  be  transported  over  railroads  and 
waterways  for  an  easily  ascertainable  price.  vSuch  a  trans- 
mission may  be  said  to  have  a  definite  efficiency,  as  for  example 
90  per  cent,  when  the  total  transportation  charges  against  a  ton 
of  coal  amount  to  10  per  cent  of  its  final  value.  From  this 
standpoint  it  is  cjuite  possible  to  transmit  power  at  this  very 
high  efficiency  even  to  the  distance  of  hundreds  of  miles.  If 
the  final  object  be  the  distribution  of  power  on  a  large  scale,  as 
from  a  great  central  station,  this  transmission  by  transporta- 
tion of  fuel  is  often  at  once  the  most  reliable  and  the  cheapest 
method. 

Transformation  of  the  fuel  energy  at  its  source  into  some 
other  form  for  the  purpose  of  transmission  is  generally  only 
justifiable,  first,  when  by  so  doing  fuel  not  available  for  trans- 
portation at  a  high  efficiency  can  be  rendered  valuable  by  trans- 
formation of  its  energy,  or  second,  when  it  is  to  be  utilized 
at  some  distant  point  in  a  manner  which  compels  a  loss  of 
efficiency  greater  than  that  encountered  in  transmission.  As 
an  example  of  the  first  condition ,  fully  one-third  of  the  coal  as 
ordinarily  mined  is  unfitted,  through  its  finely  divided  condition 
or  poor  quality,  for  transportation  over  considerable  distances. 
Its  commercial  value  is  so  small  i)er  ton  that  it  could  not  be 
carried  far  without  incurring  charges  for  carriage  amounthig 
to  a  large  part  of  its  value.     Hence,  every  coal  mine  accumu- 


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GENERAL  CONDITIONS  OF  POWER   TRANSMISSION.    33 

lates  a  mountainous  culm  pile  that  is  at  present  not  only 
valueless  but  cumbers  the  ground.  This  waste  product  could 
sometimes  be  very  profitably  employed  in  generating  power 
which  could  be  transmitted  at  a  relatively  very  high  efficiency 
and  sold  at  a  good  price. 

A  specimen  of  the  second  kind  may  be  found  in  the  somewhat 
rare  case  of  power  which  must  be  used  in  small  units  scattered 
over  a  considerable  territory,  so  that  they  could  be  replaced 
with  a  great  gain  in  efficiency  by  a  single  large  generating 
station.  Such  a  state  of  affairs  might  be  found  in  certain 
mining  regions  where  coal  and  iron  mines  are  interspersed. 
This  must  not  be  confounded  with  the  very  ordinary  case  of 
distributing  energy  from  a  central  station  to  various  scattered 
points,  for  we  are  here  considering  only  the  original  source  of 
the  fuel. 

When  an  extensive  distribution  of  energy  from  a  power 
station  is  contemplated,  electrical  or  similar  transmission  of 
power  to  that  station  is  generally  economical  only  on  the  condi- 
tion above  expressed,  of  using  fuel  otherwise  valueless,  since  the 
facilities  for  transportation  to  points  at  which  power  distribu- 
tion on  a  large  scale  would  be  profitable,  are  generally  good 
and  fairly  cheap.  All  this  applies  to  piping  gas  or  petroleum 
as  well  as  to  hauling  coal,  with  the  difference  that  neither  gas 
nor  petroleum  has  any  waste  corresponding  to  culm,  and  hence 
the  transportation  of  each  of  them  becomes  a  process  entirely 
comparable  with  the  transmission  of  energy  and  directly  com- 
peting therewith.  It  has  even  been  proposed  to  pipe  coal  dust 
by  pneumatic  power  for  fuel  purposes. 

Water-power  is  by  no  means  always  cheaper  than  fuel,  but  as 
a  general  rule  it  is,  and  by  such  an  amount  that  it  can  be 
transformed  into  electrical  energy  and  transmitted  to  at  least 
a  moderate  distance  without  losing  its  economic  advantage. 
It  therefore  is  usually  the  cheapest  source  from  which  to  derive 
power  for  general  distribution  on  a  large  scale. 

It  is  very  difficult  to  give  a  clear  idea  of  the  relative  cost  of 
steam  and  water-power,  for  while  the  one  can  be  predicted  for 
any  given  place  with  fair  accuracy,  the  other  is  subject  to 
immense  variations.  Once  established,  a  water-power  plant 
can  be  operated  very  cheaply,  but  the  cost  of  developing  the 


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34  ELECTRIC  TRANSMISSION  OF  PO\^ER, 

water-power  may  be  almost  anything,  and  each  ease  must  be 
figured  by  itself.  It  is  easy  to  obtain  estimates  of  the  cost  of 
developing  a  given  stream  and  to  form  a  close  estimate  of 
both  the  interest  charges  to  be  incurred  and  the  additional 
expense  of  repairs  and  of  operation.  The  cost  of  steam- 
power  for  the  same  conditions  can  be  accurately  estimated. 
The  details  of  such  estimates  we  will  discuss  later.  In  general, 
one  can  only  safely  say  that  the  costs  of  steam  and  water- 
power  overlap,  as  it  were,  so  that  while  the  more  easily 
developed  water-powers  are  cheaper  sources  of  energy  than 
fuel  at  any  ordinary  price,  there  are  many  cases  in  which  the 
great  cost  of  development  of  difficult  water-powers  prohibits 
competition  with  steam  except  where  fuel  is  very  dear.  Much 
depends  on  the  topography  of  the  country,  the  amount  and 
reliability  of  the  available  head  of  water,  the  price  at  which 
water  rights  can  be  obtained  and  various  other  local  conditions. 
To  utilize  the  normal  minimum  power  of  a  stream  is  gener- 
ally comparatively  easy,  while  so  to  take  account  of  high  water 
as  to  obtain  nearly  the  full  continuous  working  power  of  the 
stream  often  means  great  added  expense  for  storage  capacity 
and  works  to  control  and  regulate  the  flow. 

In  addition  we  have  to  consider  two  distinct  phases  of  the 
comparative  cost  —  first,  the  cost  of  steam  and  water  as  prime 
movers  for  a  source  of  power  to  be  distributed,  and  second, 
the  relation  between  these  costs  and  that  of  steam-power  at 
the  points  where  the  distribution  takes  place. 

(liven  a  proper  source  of  energy,  there  is  vast  variety  in  the 
character  of  the  work  of  transmission  and  distribution  that  is 
to  be  undertaken.  In  the  first  place,  the  point  of  utilization 
may  be  distant  anywhere  from  a  few  hundred  feet  to  many 
miles,  and  at  that  point  the  object  may  be  the  delivery  of 
mechanical  power  in  a  single  unit,  in  one  or  several  groups  of 
allied  units,  in  one  or  several  widely  scattered  groups,  or 
finally  for  transformation  into  some  other  form  of  energy  in 
the  most  direct  way  possible. 

There  is  no  single  method  of  power  transmission  which 
meets  in  the  best  possible  manner  all  these  widely  varying  con- 
ditions. Although  electrical  transmission  is  the  most  general 
solution  of  the  difficult  problem  in  hand,  there  are  cases  in 


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GENERAL  CONDITIONS  OF' POWER  TRANSMISSION.    86 

which  other  methods  are  preferable  and  should  be  adopted. 
Those  besides  electric  transmission  which  have  come  into  con- 
siderable use  are  the  following: 
I.   Wire  Rope  Transmission. 
II.    Hydraulic  Transmission. 

III.  Compressed  Air  Transmission. 

IV.  Gas  Transmission. 

It  will  be  well  to  look  into  the  distinguishing  characteristics 
of  these  and  their  relation  to  electrical  transmission,  with  the 
purpose  of  finding  the  advantages  and  limitations  of  each,  so 
that  the  proper  economic  sphere  of  each  may  be  determined, 
before  taking  up  the  electrical  work  which  forms  the  main 
subject  of  this  volume.  Each  method  will  be  found  to  have 
its  own  legitimate  place. 

I.  The  transmission  of  power  by  wire  ropes  is  merely  a  ver}' 
useful  extension  of  the  ordinary  process  of  belting.  Belts  are 
made  of  material  which  will  not  stand  exposure  to  the  weather, 
and  which  being  of  low  tensile  strength  is  heavy  and  bulky  in 
proportion  to  the  power  transmitted.  The  advantage  of  wire 
rope  over  belting  lies  in  its  high  tensile  strength  and  freedom 
from  deterioration  when  used  out  of  doors.  To  gain  the 
fullest  benefit  from  these  properties  it  is  necessary  to  use 
light  ropes  driven  at  high  speed. 

It  should  be  borne  in  mind  that  the  power  transmitted  by 
anything  of  the  nature  of  belting  depends  directly  on  the 
speed  and  the  amount  of  ])ull  exercised.  If  the  force  of  the 
pull  is  100  pounds  weiglit  and  the  speed  of  belt  or  rope  is 
4,000  feet  per  minute,  the  amount  of  power  transmitted  is 
400,000  foot-pounds  per  minute  or  (since  1  horse-power  is  33,000 
foot-pounds  per  minute)  about  12  IIP.  The  greater  the  speed, 
the  more  power  transmitted  with  the  same  pull,  or  the  less 
the  pull  for  the  same  power.  Wire  ro})e  can  be  safely  run 
at  a  considerably  higher  sj)eed  than  belting  and  is  much  stronger 
in  proportion  to  its  size  and  weight.  It  does  not  often  replace 
belting  for  ordinary  work,  for  the  reason  that  owing  to  its 
small  size  it  does  not  grip  ordinary  pulleys  anywhere  nearly 
in  proportion  to  its  strength.  Hence,  to  best  take  advantage 
of  its  ability  to  transmit  large  powers,  the  rope  speed  nuist  be 
high  and  the  pulleys  unusually  large  in  diameter  to  give  sufE- 


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36  ELECTRIC  TRANSMISSION  OF  POWER. 

cient  surface  of  contact.  Such  large  wheels  are  inconvenient 
in  most  situations,  and  as  the  alternative  is  a  number  of  ropes 
which  are  troublesome  to  care  for,  rope  driving  save  for  outdoor 
work  is  rather  uncommon. 

A  typical  rope  transmission  is  shown  diagrammatically  in 
Fig.  10.  Here  A  and  B  are  two  wheels,  usually  of  cast  iron, 
generally  from  5  to  15  feet  in  diameter  and  with  deeply  grooved 


Fio.  10. 


rims.  They  are  connected  by  a  wire  rope  perhaps  from  ^  inch 
to  li  inch  in  diameter,  which  serves  to  transmit  the  power  as 
the  wheels  revolve.  The  rope  speed  is  usually  from  3,000  to 
5,000  feet  per  minute,  sometimes  as  high  as  6,000.  The  distance 
between  the  centres  of  A  and  B  may  be  anything  required  by 
the  conditions  up  to  four  or  even  five  hundred  feet.  Greater 
distances  are  seldom  attempted  in  a  single  span,  as,  if  the  rope 
is  not  to  be  overstrained  by  its  own  weight,  it  must  be  allowed 
to  sag  considerably,  compelling  the  pulleys  to  be  raised  to 
keep  it  clear  of  the  ground,  and  subjecting  it  to  danger  from 
swaying  seriously  by  reason  of  wind  pressure  or  other  acci- 
dental causes. 

The  rope  employed  is  of  special  character.     The  material  is 
the  best  charcoal  iron  or  low  steel,  and  the  strands  are  usually 


Fig.  11. 


laid  around  a  hemp  core  to  give  added  flexibility.  The  rope 
generally  employed  in  this  country  is  of  six  strands  with  seven 
wires  per  strand,  and  is  shown  in  cross-section  in  Fig.  11. 
Even  with  the  hemp  core  there  is  still  in  an  iron  rope  sufficient 
resistance  to  bending   to   make   the   use  of   pulleys  of  large 


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GENERAL  CONDITIONS  OF  POWER   TRANSMISSION.    37 

diameter  necessary.  Sometimes  each  separate  strand  is  made 
A\ith  a  hemp  core,  or  is  composed  of  nineteen  small  wires 
instead  of  seven  larger  ones,  to  increase  the  flexibility  and  to 
make  it  possible  to  use  smaller  sheaves  and  drums,  as  in  hoist- 
ing machinery. 

Steel  rope  is  slightly  more  costly  than  iron,  but  gives  greater 
durability.  The  wheels  on  which  these  ropes  run  are  fur- 
nished with  a  deep  groove  around  the  circumference,  pro- 
vided with  a  relatively  soft  packing  at  the  bottom  on  which 
the  rope  rests,  and  which  serves  to  increase  the  grip  of  the 
rope  and  to  decrease  the  wear  upon  it.  Fig.  12  shows  a  section 
of  the  rim  of  such  a  wheel.     The  bushing  at  the  bottom  of  the 


Fig.  12. 

groove,  upon  which  the  rope  directly  bears,  has  been  made  of 
various  materials,  but  at  present,  leather  and  especially  prepared 
rubber  are  in  most  general  use.  The  small  pieces  of  which 
the  bushing  is  composed  are  cut  to  shape  and  driven  into  the 
dovetailed  recess  at  the  bottom  of  the  groove.  The  bushings 
have  to  be  replaced  at  frequent  intervals,  and  the  ca5Ies  them- 
selves have  an  average  life  of  not  nmch  over  a  year. 

When  a  straightaway  transmission  of  a  few  hundred  feet  is 
necessary,  when  the  power  concerned  is  not  great,  and  the  size 
of  the  pulleys  is  not  a  serious  inconvenience,  this  transmission 
by  wire  rope  is  both  very  cheap  and  enormously  efficient. 
No  other  known  method  can  compete  with  it  within  these 
somewhat  narrow  limitations.     For  a  span  of  ordinary  length 


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38  ELECTRIC   TRANSMISSION  OF  POWER. 

and  the  usual  rope  speeds,  the  efficiency  has  been  shown  by 
experiment  to  be  between  96  and  97  per  cent.  At  a  dis- 
tance of  four  to  five  hundred  feet  the  weight  and  sag  of  the 
rope  becomes  a  very  serious  inconvenience,  and  the  arrange- 
ment has  to  be  modified.     Perhaps  the  most  obvious  plan  is 


Fio.  13. 

to  introduce  a  sheave  to  support  the  slack  of  the  cable,  as 
shown  in  Fig.  13. 

On  longer  spans  several  sheaves  become  necessary,  and  both 
the  slack  and  the  tight  portions  of  the  cable  need  such  sui)port. 
In  cable  railway  work,  the  most  familiar  instance  of  power 
transmission  by  wire  ro[)es,  numerous  sheaves  have  to  be 
employed  to  keep  the  cable  in  it«  working  position  in  the 
somewhat  contracted  conduit.  These  reduce  the  efficiency  of 
the  system  considerably,  so  that  the  power  taken  to  run  the 
cable  light  is  often  greater  than  the  net  power  transmitted. 
In  aerial  cable  lines  multiple  sheaves  are  seldom  used,  and  the 
more  usual  procedure  is  to  subdivide  the  transmission  into 
several  independent  spans,  thus  lessening  swaying  and  sagging 
as  well  as  the  length  of  rope  that  nuist  be  discarded  in  case  of  a 
serious  break.  This  device  is  shown  in  Fig.  14.  It  employs 
intermediate   pulley   stations   at   which   are   installed   double 


Fkj.  14. 

grooved  pulleys  to  accommodate  the  separate  cables  that 
form  the  individual  spans.  Such  a  pulley  is  shown  in  section 
in  Fig.  15.  The  spans  may  be  three  or  four  hundred  feet 
long;  as  soon  as  the  length  gets  troublesome  another  pulley 
station  is  employed.  There  is  necessarily  a  certain  small  loss  of 
energy  at  each  such  station.  This  is  approximately  proportional 
to  the  number  of  times  the  rope  passes  over    a  pulley.     From 


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GEXERAL  COXDITIOXS  OF  POWER   TRAXSMISSIOX.    39 

the  best  experimental  data  availalile  the  officioncy  of  a  rope 
transmission  extended  by  separate  spans  is  nearly  as  follows: 


Number  of  spans  .  . 
Per  cent  efficiency  . 


..123460789      10 
.   .96     .94     .93     .91     .89     .87     .8(5     .86     .84     .82 


These  figures  are  taken  to  the  nearest  per  cent  and  are  for 
full  load  only.  At  half-load  the  loss  in  each  case  would  be 
doubled.      For  instance,  a  10-span   transmission  at  half-load 


Fio.  16. 


would  give  about  64  per  cent  efficiency.  The  pulley  stations 
consist  of  the  double-grooved  wheel  before  mentioned  mounted 
on  a  substantial  and  rather  high  pedestal  or  frame-work. 
In   this   country  a  timber  frame  is  generally   used;  abroad 


Fig.  16. 

a  masonry  pier  is  more  common.  A  convenient  fonii  of 
frame-work  is  shown  in  Pig.  16.  An  idea  of  its  dimensions  may 
be  gained  from  the  fact  that  the  wheel  is  likely  to  be  6  to  10 
feet  in  diameter. 

It  is  interesting  to  note  that  the  efficiency  just  given  for  a  10- 


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40 


ELECTRIC  TRANSMISSION  OF  POWER, 


span  transmission  at  full  load  is  quite  nearly  the  same  as  would 
be  obtained  from  an  electrical-power  transmission  at  moderate 
voltage  over  the  same  distance,  assuming  a  unit  of  say  50  HP 
or  upward.  The  first  cost  of  the  latter  would  be  considerably 
higher  than  that  of  the  rope  transmission,  but  the  repairs 
would  certainly  be  much  less  than  the  replacements  of  cable, 
bringing  the  cost  per  HP  at  full  load  to  about  the  same  fig- 
ure by  the  two  methods. 

From  actual  tests  of  electrical  apparatus  we  have  the  fol- 
lowing efficiency  for  a  transmission  of  50  HP  5,000  feet,  assum- 
ing 2,000  volts  and  2  per  cent  line  loss,  which  would  require 


Fig.  17. 

a  wire  less  than  one-fourth  of  an  inch  in  diameter.  Efficiency 
at  full  load  81  per  cent,  at  half -load  72.  These  values  are 
lower  than  those  attainable  with  machinery  of  the  most  recent 
type,  which  should  give  at  least  86  per  cent  at  full  load  and 
80  per  cent  at  half-load  for  the  complete  transmission,  which 
beats  out  rope  transmission  at  a  distance  much  less  than 
5,000  feet.  Except  at  full  load  the  electrical  transmission  has 
a  very  material  advantage.  This  advantage  would  be  greatly 
increased  if  the  transmission  were  in  anything  but  a  straight 
line.  An  electric  line  can  be  carried  around  any  number  of 
comers  without  loss  of  efficiency,  while  a  rope  transmission 
cannot.     If  it  becomes  needful  to  change  the  direction  of  a 


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GENERAL  CONDITIONS  OF  POWER   TRANSMISSION.    44 

rope  drive,  it  is  done  at  a  station  provided  with  a  pair  of  rope 
wheels  connected  by  bevel  gears  set  at  any  required  angle. 
Fig.  17  shows  such  a  station  in  diagram.  The  loss  of  energy 
in  such  a  pair  of  bevel  gears  amounts  to  from  7  to  10  per 
cent,  more  often  the  latter.  The  bevel  gears  may  be  avoided 
by  a  sheave  revolving  in  a  horizontal  plane,  and  carrying 
the  turn  in  the  cable,  but  while  this  arrangement  is  tolerably 
efficient,  it  greatly  decreases  the  life  of  the  rope. 

From  what  has  been  said,  it  will  be  seen  that  while  cable 
transmission  is  for  short  distances  in  a  straight  line  both  cheap 
and  very  efficient,  at  2,000  to  3,000  feet  it  is  equalled  and  sur- 
passed in  efficiency  by  electric  transmission,  with  lesser  main- 
tenance although  greater  first  cost.  The  steel  rope  for  a  50 
HP  transmission  of  5,000  feet  would  cost  about  $400,  and 
replacement  brings  a  considerable  charge  against  each  HP 
delivered.  If  the  transmission  is  not  straightaway,  or  if 
branches  have  to  be  taken  off  en  rotUCy  the  efficiency  of  the 
s)rstem  is  considerably  reduced  by  gear  stations,  while  even 
aside  from  these  the  efficiency  is  high  only  at  or  near  full  load. 
But  the  general  simplicity  and  cheapness  of  cable  transmission 
have  made  it  a  favorite  method,  and  there  have  been  many  such 
installations,  some  of  them  of  a  quite  elaborate  character. 
Most  of  them  are  small,  since  the  amount  of  power  that  can 
be  transmitted  by  a  single  rope  is  limited  to  250  or  300  HP. 
Ropes  suited  to  a  larger  power  are  too  heavy  and  inflexible; 
1|  inch  is  about  the  greatest  practicable  diameter  of  cable, 
and  even  this  requires  pulleys  between  15  and  20  feet  in  diam- 
eter for  its  proper  operation.  Besides,  even  at  moderate 
distances  the  rope  transmission  suffers  in  wet  or  icy  weather, 
so  that  at  anywhere  nearly  equal  costs  the  electrical  drive 
is  to  be  preferred  save  in  the  simplest  cases. 

I'nder  all  circumstances  the  need  of  replacing  the  cables 
every  year  or  so  causes  a  high  rate  of  maintenance.  The  fol- 
lowing table,  giving  the  sizes  of  iron-wire  cables  and  pulleys 
necessary  for  transmitting  various  amounts  of  power,  will  help 
to  give  a  clearer  idea  of  the  conditions  of  cable  transmission 
and  aid  in  defining  its  limited  but  useful  sphere.  Speed  is  given 
in  revolutions  per  minute,  and  pulley  diameter  is  the  smallest 
permissible.    These  figures  are,  as  will  readily  be  seen,  for  rope 


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42 


ELECTRIC   TR.lSSMfSSlOX  OF  POWER. 


Diameter  of  Rope. 

Speed. 

Diameter  of  Pulley. 

HP. 

V 

150 

6' 

26 

9  '/ 

1« 

140 

r 

3o 

r 

140 

8' 

45 

\\" 

100 

10' 

85 

80 

12' 

100 

80 

14' 

140 

w 

80 

14' 

150 

speeds  of  not  far  from  3,0()0  feet  per  minute.  This  ran  fre- 
quently be  safely  raised  to  5,0()0  with  somewhat  larger  pulleys 
than  those  given  and  increased  revolutions,  while  for  steady 
loads  the  tension  can  be  slightly  augmented  without  danger. 
So  while  the  figures  given  are  those  suitable  for  ordinary 
running  with  a  good  margin  of  capacity,  the  HP  given  can  be 
nearly  doubled  when  all  conditions  are  favorable. 

But  from  all  these  figures  it  is  sufficiently  evident  that 
rope  transmission  is  very  limited  in  its  applicability  and  is 
not  at  all  suited  to  work  of  distribution  in  small  units.  For 
a  good  many  years,  however,  a  wire-rope  transmission,  now 
practically  sup)erseded  by  electric  driving,  was  operated  at 
SchafThausen  on  the  falls  of  the  Rhine.  The  power  station 
delivered  more  than  600  HP  to  a  score  of  consumers  over 
distances  of  half  a  mile  or  so.  There  were  two  bevel-gear 
stations,  and  on  the  average,  five  cable  spans  between  the 
power  station  and  the  consumer,  so  that  the  efficiency  even 
at  full  load  wa«  somewhere  between  60  and  70  per  cent  and 
ordinarily  very  much  less.  Nevertheless,  in  default  of  any 
better  means  of  transmission  at  the  time  of  installation,  some 
twenty  years  since,  the  plant  did  fairly  successful  work,  even 
from  a  commercial  standpoint.  In  this  country  the  system 
is  very  little  used  save  for  short  straight  runs  between  building 
and  building  across  streets,  for  instance. 

II.  Noting,  then,  that  cable  transmission  does  excellent  work 
in  its  proper  place,  but  is  unsuited  for  the  distribution  of  power 
or  for  transmissions  of  anything  save  the  simplest  sorts,  we 
may  pass  to  the  hydraulic  method  of  transmitting  and  distrib- 
uting power.  This  in  its  crude  form  of  small  water-motors 
attached  to  ordinary  city  mains  is  very  familiar,  but  nothing 


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GENERAL  COSDITfOXS  OF  POWER   TRAXSMISSIOX.    43 

more  extensive  has  been  attempted  in  this  country.  Al)road 
there  are  a  number  of  hydraulic  power  plants  specially  intended 
for  the  distribution  of  power  for  general  use,  and  the  method  is 
one  which  has  been  fairly  successful.  There  are  two  distinct 
types  of  hydraulic  plant,  one  utilizing  such  pressure  as  is 
available  naturally  or  by  pumping  to  reservoirs,  the  other 
employing  very  high  artificial  pressures,  up  to  75()  pounds  per 
square  inch,  and  used  only  for  special  purposes. 

There  are  s(unewhat  extensive  works  of  the  former  kind  at 
Zurich,  Geneva,  and  Genoa,  the  effective  head  of  water  being 
in  each  case  not  far  from  5()0  feet.  In  each  case  the  power 
business  has  been  an  outgrowth  of  the  nuuiicipal  water-supply 
system.  At  Zurich  and  Cieneva  elevated  reservoirs  are  supplied 
by  pumping  stations  driven  by  water-power.  At  Genoa  the 
head  is  a  natural  one,  20  miles  from  the  city,  and  nuich  of  the 
fall  is  utilized  IS  miles  from  (ienoa  in  driving  the  fine  constant 
current  electric  ])lant  described  elsewhere  in  this  volume. 

At  Zurich  there  is  in  addition  to  the  ordinary  low  pressure 
water  system  a  special  high  service  reservoir  supplying  power 
to  a  large  electric  station  and  to  small  consumers.  Water  is 
pumped  6,000  feet  into  this  reservoir  through  an  18-inch  main, 
and  the  total  power  service  from  both  systems  is  something 
like  500  HP,  reckoned  on  a  ten-hour  basis.  The  price  charged 
is  from  $37  to  $80  per  HP  per  year. 

The  Geneva  plant  is  on  a  much  larger  scale,  the  total  turbine 
capacity  being  about  4,500  HP.  Here,  as  at  Zurich,  there  are 
two  sets  of  mains,  one  at  nearly  200  feet  head,  the  other  at 
about  450.  Each  supplies  water  for  both  ]>ower  and  general 
purposes.  The  high  pressure  service  reservoir  is  about  2^ 
miles  from  the  city,  and  the  working  pressure  is  supplied 
indifferently  from  this  or  from  the  pumps  direct.  There  is  an 
electric  light  plant  with  600  HP  in  turbines  driven  by  the 
pressure  water,  and  a  large  number  of  smaller  consumers. 
Water  is  supplied  to  the  electric  light  company  for  as  low  as 
$15  per  HP  per  year. 

Both  these  installations  are  extensions  of  the  city  water 
service,  and  have  done  excellent  work.  Operated  in  this  way 
the  economic  conditions  are  somewhat  different  from  those  to 
be  found  in  a  hydraulic  plant  established  by  private  enterprise 


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44 


ELECTRIC   TRANSMISSION  OF  POWER. 


for  power  only.  An  inquiry  into  the  efficiency  of  such  a 
system  may  be  fairly  based  on  the  facts  given.  At  Zurich,  for 
example,  the  efficiency  from  turbine  shaft  to  reservoir  cannot 
well  exceed  75.  The  distributing  mains  must  involve  a  loss 
of  not  less  than  10  per  cent,  while  the  motors  cannot  be 
counted  on  for  an  efficiency  of  over  .75.  The  total  efficiency 
from  turbine  shaft  to  motor  shaft  is  then  about  .75  x  .75  X  .90 
=  50.6  per  cent.  The  character  of  the  motors  has  an  impor- 
tant influence  on  the  economy  of  the  system,  particularly  at 
low  loads.     The  motors  most  used  particularly  for  small  powers 


r~\ 


Km;,  is. 

are  oscillating  water  engines  of  the  type  shown  hi  Fig.  18. 
The  form  shown  is  made  by  Schmid  of  Zurich.  It  possesses,  in 
common  with  all  others  of  similar  construction,  the  undesirable 
property  of  taking  a  uniform  amount  of  water  at  uniform  speed , 
quite  irrespective  of  load.  The  mechanical  efficiency  falls  off 
like  that  of  a  steam  engine,  friction  being  nearly  constant. 
Better  average  results  are  secured  with  impulse  turbines  (see 
Chapter  IX)  of  which  the  efficiency  varies  but  little  as  the  load 
falls  off,  or  for  high  rotative  speeds  wdth  impulse  wheels  like  the 
Pel  ton,  shown  in  Fig.  19,  as  adapted  for  motors  of  moderate 
power.     At  half-load,  i.e.,  half  flow,  the  losses  in  distributing 


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GENERAL  CONDITIONS  OF  POWER  TRANSMISSION.    46 

mains  would  be  reduced  to  about  one-third,  while  the  efficiency 
of  the  engine  motors  would  certainly  not  be  lowered  by  less 
than  5  per  cent.  The  total  half-load  efficiency  would  then  be 
.75  X  .97  X  .70  =  50.9  per  cent,  actually  a  trifle  higher  than 
at  full  load.  This  apparently  remarkable  prop)erty  is  shared 
by  all  transmissions  wherein  the  transmission  loss  proper  is 
fairly  large. 

The  second  type  of  hydraulic  distribution  of  power  is  that 
at  very  high  pressures  and  employing  a  purely  artificial  head. 
The  pressures  involved  are  usually  700  to  800  pounds  per  square 


FlO.  19. 


inch,  and  a  small  amount  of  storage  capacity  is  gained  by 
employing  .what  are  known  as  hydraulic  accumulators,  fed  by 
the  pressure  pumps.  These  accumulators  are  merely  long 
vertical  cylinders  adapted  to  withstand  the  working  pressure, 
which  is  kept  up  by  a  closely  fitting  and  enormously  heavy 
piston.  The  distribution  of  power  is  by  iron  pipes  leading  to 
the  various  water  motors.  This  high  pressure  water  system 
is  a  device  almost  peculiar  to  England,  and  has  been  slow  in 
making  headway  elsewhere.  Its  especial  advantage  is  in  con- 
nection with  an  exceedingly  intermittent  load,  such  as  is 
obtained  from  cranes,  hoists,  and  the  like.  This  is  for  the 
reason  that  with  a  low  average  output  a  comparatively  small 


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46  ELECTRIC  TRAXSMISSION  OF  POWER, 

engine  and  pump  working  continuously  at  nearly  uniform  load 
can  keep  the  accumulators  charged,  while  the  rate  of  output 
of  the  accumulators  is  enormous  in  case  of  a  brief  demand  for 
very  great  power. 

Power  plants  on  this  hydraulic  accumulator  system  are  in 
operation  in  the  cities  of  London,  Liverpool,  Hull,  and  Bir- 
mingham, p]ngland,  and  at  Marseilles,  France.  The  Lon- 
don plant  is  the  most  important  of  those  mentioned,  con-, 
sisting  of  three  pumping  and  accumulator  stations  and  about 
60  miles  of  mains.  The  total  number  of  motors  operated  was 
in  1892  about  1,700.  The  charges  are  by  meter,  and  are  based 
on  intermittent  work,  being  quite  prohibitive  for  continuous 
service  —  from  $200  to  $500  per  effective  HP  per  year  of  3,000 
hours.  The  largest  accumulators  have  pistons  20  inches  in 
diameter  and  23  feet  stroke,  giving  a  storage  capacity  of  only 
24  horse-power-hours  each.  While  very  convenient  for  the 
supply  of  power  for  intermittent  service  only,  this  system,  like 
hydraulic  supply  at  low  pressure,  is  rather  inefficient,  the  more 
so  as  it  has  been  found  advisable  to  employ  hydraulic  motors 
of  the  piston  type,  althougli  special  Pelton  motors  have  been 
used  in  some  cases. 

Any  hydraulic  system  suffers  severely  from  the  inefficiency 
of  pump  and  motors  and  from  loss  of  head  in  the  pipes.  The 
amount  of  power  that  can  be  transmitted  in  the  mains  is  (juite 
limited,  since  the  permissible  velocity  is  not  large.  About 
3  feet  per  second  is  customary  —  more  than  this  involves 
excessive  friction  and  danger  from  hydraulic  shock.  At  this 
speed  a  pipe  about  2  feet  in  diameter  is  necessary  to  transmit 
5(K)  HP  under  500  feet  head.*  The  power  delivered  increases 
directly  with  the  head,  but  as  the  pressure  increases  the  largest 
practicable  size  of  pipe  decreases,  and  on  the  high  pressure 
systems  nothing  larger  than  12  inches  has  been  attempted,  and 
even  this  requires  the  use  of  solid  drawn  steel 

Whatever  the  size  of  pipe,  the  loss  in  head  is  quite  nearly 
inversely  as  the  diameter  and  directly  as  the  square  of  the 
velocity.  Even  for  high  pressure  systems  this  loss  is  by  no 
means  negligible,  since  the  pipes  used  are  rather  small. 

The  following  table  gives  the  loss  of  head  in  feet  per  100  feet 
*  Cost  per  mile  laid  in  average  uiipaved  ground  about  §15,000. 


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GEXERAL  CONDITIOSS  OF  POWER  TRAXSMISSIOX.    47 

of  pipe  and  at  a  uniform  velocity  of  3  feet  per  second.  This 
applies  to  pipe  in  good  average  condition.  When  the  pipe  is 
new  and  quite  clean,  the  losses  may  he  slightly  less.  If  the 
pipe  is  old  and  incrusted,  the  above  losses  may  be  nearly 
doubled.  Hends  and  branches  still  further  reduce  the  working 
pressure. 


Diameter      .... 

1"        2" 

1 

3" 

1 

1  '' 

5" 

ir 

7" 

1 
1  8" 

1 

w 

1 
|12" 

Loss  of  Head    .     .     . 

1 
4.89  1  2.44 

1 

1.62 

1.22 

.98 

.81 

1         '         1 
.70  ,  .61     .49    .41 

_-    :    ^    -                    r    _      -^-_ 

1 

1             1                                       ! 
""  .  ~          1                        ~ 

Diameter     .... 

14"  '  1«" 

18" 

20" 

22" 

24"  1  2«" 

1 

28" 

40" 

36" 

Loss  of  Head   .     .     . 

.35      .32 

.27 

.25 

.22 

.20 

.19 

.17 

.10 

.13 

We  may  now  look  into  the  efficiency  of  these  high  pressure 
hydraulic  systems.  Of  the  mechanical  horse-power  applied  to 
the  pump  we  cannot  reasonably  hope  to  get  more  than  75  per 
cent  as  energy  stored  in  the  accunmlators.  Tests  on  the 
Marseilles  plant  have  sho\^ai  70  to  80  per  cent  efficiency  between 
the  indicated  steam  power  and  the  accunmlators,  the  former 
figure  at  the  speeds  corresponding  to  full  working  capacity.  As 
the  pumps  were  direct  acting  the  difference  between  brake  and 
indicated  HP  was  presumably  very  small.  The  motors  can  be 
counted  on  for  about  .75  efficiency,  and  the  losses  of  head  in 
the  pipes  for  any  ordinary  distribution  cannot  safely  be  taken 
at  less  than  5  per  cent.  Hence  the  full  load  efficiency  is  about 
.75  X  .75  X  .95  =  .53.  The  efficiency  at  full  load  is  thus  not 
far  from  that  of  the  low  pressure  system,  but  at  half-load  it 
suffei-s  from  the  use  of  ])iston  motors,  generally  necessary  on 
account  of  the  too  high  speed  of  rotary  motors  at  high  pressure. 
At  even  500  pounds  per  s(iuare  inch  pressure  the  normal  speed 
of  a  Pelton  wheel  of  say  20  HP  would  be  over  4,000  r.  p.  m.,  and 
could  not  be  greatly  reduced  without  seriously  cutting  down 
the  efficiency.  At  half-load  the  piston  motors  could  not  be 
relied  on  for  over  .65  efficiency,  reducing  the  total  efficiency, 
even  allowing  for  greatly  lessened  pipe  loss,  to  about  45  per 
cent.    On  the  whole,  the  hvdraulic  accimuilator  svstem  must  be 


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48  ELECTRIC  TRANSMISSION  OF  POWER. 

regarded  as  a  very  ingenious  and  occasionally  useful  freak.  It 
may  now  and  then  be  useful  as  an  auxiliary  in  the  storage  of 
energy  from  a  very  irregular  power  supply. 

The  strongest  point  of  hydraulic  transmission  is  its  ready 
adaptability  in  connection  with  water  supply  systems  for  gen- 
eral purposes.  Skilfully  installed,  as  for  instance  at  Geneva, 
it  furnishes  convenient,  reliable,  and  fairly  cheap  motive  power. 
As  a  distinct  power  enterprise  the  high  first  cost  is  against  it, 
and  the  efficiency  is  never  really  good.  All  this  applies  with 
even  greater  force  to  the  special  high  pressure  systems,  which 
suffer  from  inability  to  cope  with  continuous  work,  thus  seri- 
ously limiting  the  possible  market.  Even  for  intermittent  ser- 
vice the  charges  are  enormously  high.      ^ 

The  methods  of  power  transmission  already  mentioned  are 
then  somewhat  limited  in  their  usefulness  by  rather  well 
defined  conditions,  which  make  their  employment  advisable  in 
some  cases  and  definitely  inadvisable  in  general. 

III.  We  may  now  pass  to  the  pneumatic  method  of  transmit- 
ting power,  which  is  far  more  general  in  its  convenient  appli- 
cability than  either  of  the  others,  and  which  is  the  only  system 
other  than  electric  which  has  been  extensively  applied  in  prac- 
tice to  the  distribution  of  power  in  small  units,  although  only 
short  distances  have  been  involved  in  any  of  the  plants 
hitherto  operated,  and  the  possible  performance  at  long  dis- 
tances is  more  a  subject  of  speculation  than  of  reasonable  cer- 
tainty. Transmission  of  power  by  compressed  air  involves 
.essentially  three  elements:  An  air  compressor  delivering  the 
air  under  a  tension  of  from  50  to  100  or  more  pounds  per 
square  inch  into  a  pipe  system,  which  conveys  the  compressed 
air  to  the  varioiLs  motors.  These  motors  are  substantially 
steam  engines  in  mechanical  arrangements,  and  indeed  almost 
any  steam  engine  can  be  readily  adapted  for  use  with  com- 
pressed air.  The  compressor  itself  is  not  unlike  an  ordinary 
steam  pump  in  general  arrangement.  Its  appearance  in  the 
smaller  sizes  is  well  sho^Mi  in  diagram  in  Fig.  20.  The  system 
was  originally  introduced  about  fifty  years  ago  for  mining 
purposes,  and  owed  its  early  importance  to  its  use  in  working 
the  drills  in  the  construction  of  the  Hoosac,  Mont  Cenis,  and 
St.  Gothard  tunnels.     Since  then  it  has  come  to  be  used  on  a 


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GENERAL  CONDITIONS  OF  POWER   TRANSMISSION,    49 

very  extensive  scale  for  drilling  operations,  and  recently  has 
also  been  applied  for  the  distribution  of  power  for  general  pur- 
poses, particularly  in  Paris,  where  the  only  really  extensive 
system  of  this  kind  is  in  operation.  Its  best  field  has  been 
and  still  is  in  mining  operations  where  the  escaping  air  is  a 
welcome  addition  to  the  means  of  ventilation  and  where,  as  a 
rule,  the  distances  are  not  great. 

Transmission  of  power  by  piping  compressed  air  has  even 
for  general  distribution  certain  very  well  marked  advan- 
tages.    The  subdivision  of  the  power  can  be  carried  on  to 


Pig.  20. 

almost  any  extent,  and  the  motors  are  fairly  efficient,  simple, 
and  relatively  cheap.  In  addition,  the  power  furnished  to 
consumers  can  very  easily  be  metered.  The  loss  of  energy 
can  be  kept  within  moderate  limits,  and  the  mains  themselves 
are  not  liable  to  serioiLs  breakdowns,  although  losses  from 
leakage  are  frequent  and  may  be  large.  Finally,  the  system 
is  exceptionally  safe.  On  the  other  hand,  the  efficiency  of 
the  system,  reckoned  to  the  motor  pulleys,  is  unpleasantly 
low.  The  mains  for  a  transmission  of  any  considerable  length 
are  very  costly,  and  the  compressed  air  has  no  considerable 
use   aside   from   motive   power,   instead   of  being  applicable, 


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60  ELECTRIC  TRANSMISSION  OF  POWER. 

like  electric  or  even  hydraulic  transmission  of  power,  to  divers 
profitable  employments  quite  apart  from  the  furnishing  of 
mechanical  energy.  To  obtain  a  clearer  idea  of  the  nature 
of  these  advantages  and  disadvantages,  let  us  follow  the  process 
of  pneumatic  transmission  from  the  compressor  to  the  motor, 
looking  into  each  stage  of  the  operation  with  reference  to 
its  efficiency  and  economic  value. 

The  compressor  is  the  starting  point  of  the  operation.  Fig. 
20  shows  in  section  a  typical  direct  acting  steam  compressor, 
one  of  the  best  of  its  class.  It  consists  essentially  of  the  air 
cylinder  A  and  a  steam  cylinder  B,  arranged  in  line  and  having 
a  common  piston  rod.  The  steam  end  of  the  machine  is 
simply  an  ordinary  engine  fitted  with  an  excellent  high  speed 
valve  gear  worked  l)y  two  eccentrics  on  the  crank  shaft  of 
the  flywheels  G,  which  serve  merely  to  steady  the  action  of 
the  mechanism.  The  air  cylinder  A  is  provided  with  a  simple 
piston  driven  by  an  extension  of  the  steam  piston  rod. 

At  each  end  of  the  air  cylinder  are  automatic  poppet  valves 
E  E,  which  serve  to  admit  the  air  and  to  retain  it  during  the 
process  of  compression.  F  is  the  discharge  pipe  for  the  com-, 
pressed  air  leaving  the  cylinder.  In  the  compressor  shown 
there  are  two  steam  and  two  air  cylinders  connected  with  the 
cranks  90°  apart,  thus  giving  steady  rotation  in  spite  of  the 
character  of  the  work.  In  some  machines  the  pistons  and 
piston  rods  are  hollow  and  provided  with  means  for  maintaining 
water  circulation  through  them,  to  assist  in  cooling  the  air. 
Round  the  air  cylinder  is  a  water  jacket  shown  in  the  cut  just 
outside  the  cylinder  wall.  The  purpose  of  this  is  to  keep  the 
air,  so  far  as  pos.sible,  cool  during  compression,  and  thus  to 
avoid  putting  upon  the  machine  the  work  of  compressing  air 
at  a  pressure  enhanced  by  the  heat  that  always  is  produced 
when  air  is  compressed.  And  just  here  is  the  first  weak  point 
of  the  compressed  air  system.  However  efficient  is  the 
mechanism  of  the  compressor,  all  heat  given  to  the  air  during 
compression  represents  a  loss  of  energy,  since  the  air  loses 
this  heat  energy  before  it  reaches  the  point  of  consump- 
tion. The  higher  the  final  pressure  which  is  to  be  reached, 
the  more  useless  heating  of  the  air  and  the  lower  efficiency. 
Hence  the  water  jacket,  which,  by  abstracting  part  of  the  heat 


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GENERAL  CONDITIONS  OF  POWER   TRANSMISSION.   51 

of  compression,  aids  in  averting  needless  work  on  the  air  dur- 
ing compression.  Even  the  most  thorough  jacketing  leaves 
much  to  be  desired,  generally  leaving  the  air  discharged  at 
from  200°  to  300°  F.,  more  often  the  latter.  A  cold  water 
spray  is  often  used  in  the  compressing  cylinder.  This  is  some- 
what more  thorough  than  the  jacket,  but  is  still  rather  in- 
effective. Both  serve  only  to  mitigate  the  evil,  since  they 
cool  the  air  by  absorbing  energy  from  it,  and  at  best  cool  it 
very  imperfectly.  A  careful  series  of  investigations  by  Riedler, 
perhaps  the  best  authority  on  the  subject,  gives  for  the  efficiency 
of  the  process  of  compression  from  .49  to  .72.  These  figures, 
derived  from  seven  compressors  of  various  sizes  and  types, 
include  only  those  losses  which  are  due  to  heat,  valve  leakage, 
clearance,  and  the  like,  taking  no  account  of  fricticmal  losses 
in  the  mechanism.  These  are  ordinarily  about  the  same  as 
in  a  steam  engine,  say  10  per  cent,  so  that  the  total  efficiency 
of  a  simple  compressor  may  be  taken  as  .44  to  .65,  the  latter 
only  in  large  machines  under  very  favorable  conditions.  The 
most  considerable  recent  improvement  in  compressors  is  the 
division  of  the  compression  into  two  or  more  stages,  as  the  ex- 
pansion is  divided  in  compound  and  triple  expansion  engines. 
This  limits  the  range  of  heating  that  can  take  place  in  any 
given  cylinder,  and  greatly  facilitates  effective  cooling  of  the 
air.  Riedler  has  obtained  from  two-stage  machhies  of  his 
own  design  a  compressor  efficiency  of  nearly  .9.  Allowing  for 
the  somewhat  greater  friction  in  the  mechanism,  the  total 
efficiency  was  found  to  be  about  .76.  In  general,  then,  we  may 
take  the  total  efficiency  of  the  single  stage  compressors  usually 
employed  in  this  country  as  .5  to  .6,  very  rarely  higher,  while 
the  best  two-stage  compressors  may  give  an  efficiency  slightly 
in  excess  of  .75.  For  steady  working,  .75  would  be  an  excel- 
lent result. 

We  may  next  look  into  the  action  of  the  compressed  air  in 
the  mains.  As  in  the  case  of  w^ater,  the  frictional  resistance 
and  consequent  loss  of  pressure  vary  directly  with  the  square 
of  the  velocity  of  the  air  and  inversely  with  the  diameter  of 
the  pipe.  Hy  reducing  the  one  and  increasing  the  other,  the 
efficiency  of  the  line  may  be  increased  at  the  cost  of  a  con- 
siderable increase  in  original  outlay.     Any  attempt  to  force  the 


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ELECTRIC  TRANSMISSION  OF  POWER, 


output  of  the  main  rapidly  increases  the  losses.  At  a  working 
gauge  pressure  of  60  pounds  per  square  inch,  which  is  in  very 
frequent  use,  the  per  cent  of  pressure  lost  per  1,000  feet  of 
pipe  of  various  diameters  is  given  in  the  following  table  —  the 
velocity  being  taken  at  30  feet  per  second : 


Diameter      .     .     . 

1" 

2" 

3" 

4" 

5" 
4.37 

6" 

12" 

1 
18"    24" 

30" 

48" 

Per  cent  loss    .     . 

21.8 

10.9 

7.8 

5.45 

3.00 

1.0 

0.00 

0.6 

.33 

.25 

The  friction  in  the  pipes  is  proportionally  greater  in  small 
pipes  than  in  large,  and  this  table  is  taken  as  correct  for  the 
medium  sizes.  No  allowance  is  made  for  increase  in  velocity 
through  a  long  main,  for  leakage,  nor  for  draining  traps, 
elbows,  curves,  and  other  extra  resistances,  so  that  as  in  prac- 
tice the  larger  and  longer  mains  suffer  the  more  from  these 
various  causes,  the  table  will  not  be  found  widely  in  error  for 
ordinary  cases.  Very  large  straightaway  mains  will  give 
somewhat  better  results,  and  the  five  last  columns  of  the  table 
are  computed  from  Riedler's  experiments  on  the  Paris  air 
mains,  11 J  inches  in  diameter  and  10  miles  long.  All  losses 
are  included.  Ix)sses  in  the  air  mains  can  therefore  be  kept 
within  a  reasonable  amoimt  in  most  cases.  With  large  pipes 
and  low  velocities,  power  can  be  transmitted  with  no  more  loss 
than  is  customary  in  the  conductors  of  an  electrical  system. 
Small  distributing  pipes,  however,  entail  a  serious  loss  if  they 
are  of  any  considerable  length. 

The  motor  is  the  last  element  of  pneumatic  transmission  to 
be  considered.  Generally  it  is  almost  identical  with  an  ordi- 
nary steam  engine;  in  fact,  steam  engines  have  been  often 
utilized  for  air,  and  common  rock  drills  may  be  used  indif- 
ferently for  steam  or  air  with  sometimes  slight  changes  in  the 
packing  of  the  pistons  and  piston  rods.  Some  special  air 
motors  are  in  use  with  slight  modifications  from  the  usual 
steam  engine  type.  In  most  of  these  the  air  is  used  expan- 
sively and  at  a  fairly  good  efficiency.  Tests  l)y  Riedler  on  the 
Paris  system  show  for  the  smaller  air  motors  an  efficiency  of  as 
high  as  85  per  cent  so  far  as  the  utilization  of  the  available 


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GENERAL  CONDITIONS  OF  POWER   TRANSMISSION.    63 

energy  in  the  air  is  concerned,  or,  taking  into  account  the 
mechanical  losses,  70  to  75  per  cent.  Occasional  results  as 
low  as  50  to  60  per  cent  were  obtained  even  when  the  air  was 
used  expansively,  while  if  used  non-expansively  the  total  effi- 
ciency was  uniformly  below  40  per  cent.  Tests  on  an  adapted 
steam  engine  with  Corliss  valve  gear  gave  a  pneumatic  effi- 
ciency of  .90,  with  a  total  efficiency  of  .81.  These  figures  are 
under  more  than  usually  favorable  conditions. 

One  of  the  principal  difficulties  with  air  motors  is  freezing, 
due  to  the  sudden  expansion  of  the  compressed  air,  and  the 
congelation  of  any  moisture  carried  with  it.  It  is  quite  use- 
ful, therefore,  to  supply  to  the  motor  artificially  a  certain 
amount  of  heat,  sufficient  to  keep  the  exhaust  at  the  ordinary 
temperature,  especially  if  the  air  has  been  cooled  by  spray 
during  compression.  This  heating  process  is  very  frequently 
extended  so  as  not  only  to  obviate  all  danger  of  freezing  but  to 
add  to  the  output  of  the  air  motor  by  giving  to  the  compressed 
air  a  very  considerable  amount  of  energy.  The  air  is  passed 
through  a  simple  reheating  furnace  and  delivered  to  the  motor 
at  a  temperature  of  about  300°  Fahrenheit.  The  energy  de- 
livered by  the  motor  is  composed  of  that  actually  transmitted 
through  the  mains  pliLS  that  locally  furnished  by  the  reheater. 

The  amount  of  fuel  used  is  not  great,  usually  from  i  to  J  of 
a  pound  of  coal  per  horse-power-hour,  and  the  increase  of 
power  obtained  is  about  25  per  cent  of  that  which  would 
otherwise  be  obtained  from  the  motor.  This  means  that  the 
heat  is  very  effectively  utilized.  Reheating  is  not  a  method  of 
increasing  the  efficiency  of  the  system,  as  is  sometimes  sup- 
posed, but  a  convenient  way  of  working  a  hot  air  engine  in 
conjunction  with  an  initial  pressure  obtained  from  air  mains. 
It  increases  the  operating  expense  by  a  very  perceptible  though 
rather  small  amount,  and  gains  a  good  return  in  power.  In  so 
far  it  is  desirable,  but  it  no  more  increases  the  efficiency  of  the 
pneumatic  transmission  than  would  power  from  any  other 
source  added  to  the  power  actually  transmitted. 

We  are  now  in  a  position  to  form  a  clear  idea  of  the  real 
efficiency  of  transmission  of  power  by  compressed  air.  Taking 
the  compressor  and  motor  efficiencies  already  given,  and  assum- 
ing 10  per  cent  loss  of  energy  in  the  mains,  we  have  for  the 


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54  ELECTRIC   TRANSMISSION  OF  POWER. 

total  eflicieiicy  from  indicated  horse-power  at  the  compressor 
to  brake-horse-power  at  the  motor:  .75  x  .90  x  .80  =  .54 
for  large  two-stage  compressors  and  large  motors;  while  with 
ordinary  apparatus  it  would  be  about  .70  X  .90  x  .75  =  .47. 
At  half-load  these  figures  would  be  reduced  to  about  .45  and 
.35  respectively.  In  operating  drills,  which  are  motors  in 
which  the  air  is  used  non-expansively  and  to  which  the  air  is 
carried  considerable  distances  through  small  pipes,  the  total 
efficiency  is  almost  ahvays  below  rather  than  above  .30.  The 
efficiency  of  .54  given  above  cannot  well  be  realized  without 
recourse  to  artificial  heating  to  enable  the  air  to  be  used  expan- 
sively without  trouble  from  freezing. 

Compressed  air  has  been  mainly  used  for  mining  operations, 
where  its  entire  safety  and  its  ventilating  effect  are  strong 
point-s  in  it,s  favor.  More  rarely  it  is  employed  for  general 
power  purposes.  Of  such  use  the  Popp  compre^ssed  air  system 
in  Paris  is  the  best  and  the  only  considerable  example. 

This  great  work  started  from  a  system  of  regulating  clocks 
by  compressed  air  established  a  quarter-century  ago.  Nearly 
a  decade  later  the  use  of  the  compressed  air  for  motors  began, 
and  after  several  extensions  of  the  old  plant  the  present  station 
was  built.  It  contains  four  2,000  HP  compound  compressoi-s, 
of  which  three  are  regularly  used  and  the  fourth  held  in  r(»serve. 
The  steam  cylinders  are  triple  expansion,  worked  with  a  steam 
pressure  of  180  pounds.  The  air  pressure  is  7  atmospheres, 
and  the  new  mains  are  20  inches  in  diameter,  of  wrought  iron. 
There  are  in  all  more  than  30  miles  of  distributing  main,  most 
of  it  of  12  inches  and  imder  in  diameter.  A  very  large  number 
of  motors  of  sizes  from  a  fan  motor  to  more  than  100  HP  arc 
in  use.  Their  total  amount  runs  up  to  several  thousand  HP, 
even  though  the  majority  of  them  are  less  than  a  single  horse- 
power. Except  in  very  small  motors,  reheaters  are  used, 
raising  the  temperature  of  the  air  generally  to  between  200° 
and  300°  F.  The  efficiency  of  the  w^hole  system  from  Pro- 
fessor Kennedy's  investigations  is  about  50  per  cent  under 
very  favorable  conditions.  The  prices  charged  for  powder 
have  not  been  generally  known ,  but  are  understood  to  be  some- 
what in  excess  of  $100  per  horse-power  per  working  year. 
An  interesting  addition  to  the  apparatus  of  pneumatic  trans- 


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GENERAL  CONDITIONS  OF  POWER   TRANSMISSION.    55 

mission  has  recently  appeared.  It  is  a  modification  of  the 
ancient  "trompe/'  or  water  blast,  used  for  centuries  to  feed 
the  forges  of  Catalonia,  very  simple  in  operation  and  cheap  to 
build.  In  its  present  improved  form  it  is  known  as  the  Taylor 
Hydraulic  Air  Compressor,  and  an  initial  plant  of  very  respect- 
able size  has  been  in  highly  successful  operation  for  nearly  two 
years  past  at  Magog,  P.  Q.,  from  which  the  data  here  given 
have  been  obtained. 

The  compressing  apparatus  which  is  shown  in  Fig.  21  is  in 
principle  an  inverted  siphon  having  near  it*s  upper  end  a  series 
of  intake  tubes  for  air,  and  at  the  bend  a  chamber  to  collect 
the  air  which,  entrained  in  the  form  of  fine  bubble??,  is  carried 
down  with  the  water  column,  which  flowing  up  tlie  short  arm 
of  the  sijihon  escapes  into  the  tail  race.  In  Fig.  21,  A  Is  the 
penstock  delivering  water  to  the  supply  tank  B.  In  this 
tank  Is  the  mouth  of  the  down  tube  C,  contracted  by  the 
inverted  cone  C  so  as  to  lower  the  hydraulic  pressure  and 
allow  ready  access  of  air  from  the  surrounding  apertures. 
The  air  bubbles  trapped  in  the  water  sweep  down  C,  which 
expands  at  the  lower  end,  and  finally  enters  the  air  tank  1). 
Here  the  water  column  encounters  the  cone  K,  which  flattens 
into  a  plate  at  the  base.  Thus  spread  out  and  escaping  into 
the  air  chamber  by  the  circuitous  route  shown  by  the  arrows, 
the  air  bubbles  from  the  water  accumulate  in  the  top  of  the 
air  tank,  while  the  water  itself  rises  up  the  shaft  E,  and  flows 
into  the  tail  race  F,  The  air  in  D  is  evidently  under  a  pressure 
due  to  the  height  of  the  water  column  up  to  F,  and  quite 
independent  of  the  fall  itself,  which  consequently  may  vary 
greatly  without  aflfecting  the  pressure  of  the  stored  air,  a  very 
valuable  property  in  some  cases,  as  in  utilizing  tidal  falls. 
F'rom  D  the  compressed  air  is  led  up  through  a  pipe,  P,  for 
distribution  to  the  motors.  To  get  more  pressure,  it  is  only 
necessary  to  burrow  deeper  with  the  air  tank,  not  a  diffi- 
cult task  where  easy  digging  can  be  found.  The  fall  and  rate 
of  flow  determine  the  rate  at  which  the  air  is  compressed, 
and  contrary  to  what  might  be  supposed,  the  process  of  com- 
pression is  quite  efficient.  It  is  quite  sensitive  to  variations 
in  the  amount  of  flow,  the  efficiency  changing  rapidly  with  the 
conditions  of  inlet;  and  since  there  certainly  is  a  limit  to  the 


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ELECTRIC  TRANSMISSION  OF  POWER, 


rio.  21, 


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GENERAL  CONDiriONS  OF  POWER   TRANSMISSIOX.    57 

amount  of  air  that  can  be  entrained  in  a  given  volume  of 
water,  the  process  is  likely  to  work  most  efficiently  at  moderate 
heads  and  with  large  volumes  of  water.  In  the  Magog  com- 
pressor about  4  cubic  feet  of  water  are  required  to  entrain 
1  cubic  foot  of  air  at  atmospheric  pressure,  and  it  is  open  to 
question  as  to  how  far  this  ratio  could  be  improved.  This 
ratio,  too,  would  be  changed  for  the  w^orse  rapidly  in  attempt- 
ing high  compression,  so  that  the  Magog  results  probably  rep- 
resent, save  for  details,  very  good  working  conditions.  The 
dimensions  of  the  Magog  apparatus  are  given  in  the  accom- 
panying table,  which  is  followed  by  the  details  of  one  of  the 
tests  made  by  a  very  competent  body  of  engineers. 
The  general  dimensions  of  the  compressor  plant  are: 

Supply  penstock 60  inches  diameter 

Supply  tank  at  top 8  feet  diameter  by  10  feet  high 

Air  inlets  (feeding  numerous  small  tubes) 34  2-inch  pipes 

Down  tube 44  inches  diameter 

Down  tube  at  lower  end 60  inches  diameter 

Length  of  taper  in  down  tube,  changing  from  44-iuch  to 

60-inch  diameter 20  feet 

Air  chamber  in  lower  end  of  shaft  16  feet  diameter 

Total  depth  of  shaft  below  normal  level  of  head  water about  150  feet 

Normal  head  and  fall about  22  feet 

Air  discharge  pipe 7  inches  diameter 

Flow  of  water,  cubic  feet,  minute  4292. 

Head  and  fall  in  feet  19.509 

Gross  water  HP 158.1 

Cubic  feet  compressed  air  per  minute,  reduced  to  atmos- 
pheric pressure 1 148. 

Pressure  of  compressed  air,  lbs 53.3 

Pressure  of  atmosphere,  lbs 14.41 

Effective  work  done  in  compressing  air,  HP   111.7 

Efficiency  of  the  compressor,  per  cent    70.7 

Temperature  of  external  air,  Fahr 65.2 

Temperature  of  water  and  compressed  air,  Fahr 66.6 

Moisture  in  air  entering  compressor,  per  cent  of  saturation    68. 

Moisture  in  air  after  compression,  per  cent  of  saturation    36. 

The  efficiency  given  is  certainly  most  satisfactory,  being 
quite  as  high  as  could  be  attained  by  a  compound  compressor 
of  the  best  constniction  driven  by  a  turbine,  and  for  the  head 
in  question  at  a  very  much  lower  cost.     It  is  probable  that 


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58  ELECTRIC  TRANSMISSION  OF  POWER. 

the  test  given  does  not  represent  the  best  that  can  be  done  by 
this  method,  and  the  indications  are  that  within  a  certam, 
probably  somewhat  limited,  range  of  heads  the  hydraulic  com- 
pressor will  give  as  compressed  air  a  larger  proportion  of  the 
energy  of  the  water  than  any  other  known  apparatus.  Just 
what  its  limitations  are,  remains  to  be  discovered,  but  several 
plants  are  now  under  construction  which  will  throw  consider- 
able light  upon  the  subject.  ^ 

In  certain  cases  the  power  of  getting  compre.ssed  air  direct 
from  hydraulic  power  by  means  of  a  simple  and,  under  favor- 
able conditions,  cheap  form  of  apparatus,  is  very  valuable, 
and  while  it  is  unlikely  to  change  radically  the  status  of  pneu- 
matic transmission,  it  is  an  important  addition  to  available 
engineering  methods.  As  in  most  pneumatic  plants,  the 
Magog  installation  is  worked  in  connection  with  reheaters. 
A  similar  plant  on  a  somewhat  larger  scale  is  in  successful 
operation  near  Norwich,  Conn. 

IV.  In  point  of  convenience  and  efficiency,  compressed  air 
is  nearer  to  electricity  for  the  distribution  of  power  over  large 
areas  than  any  other  method.  The  only  other  system  that 
approaches  them  is  the  transmission  of  gaseous  fuel  for  use 
in  internal  combustion  engines.  At  equal  pressures  one  can 
send  through  a  given  pipe  twenty  times  as  much  energy  stored 
in  gas  as  in  air.  A  good  air  motor  requires  about  450  cubic 
feet  of  air  at  atmospheric  pressure  per  indicated  HP  hour, 
while  a  gas  engine  will  give  the  same  power  on  a  little  over  20 
cubic  feet  of  gas.  But  the  cases  wherein  the  distribution  of 
gas  would  be  desirable  in  connection  with  a  transmission  over 
a  long  line  of  pipe  are  comparatively  few.  Particularly  this 
system  has  no  place  in  the  development  of  water-powers,  the 
most  important  economic  function  of  electrical  transmission. 
Nevertheless  it  must  be  admitted  that  for  simple  distribution 
of  power  a  well-designed  fuel  gas  system  is  a  formidable  com- 
petitor of  any  other  method  yet  devised,  particularly  in  the 
moderate  powers  —  say  from  5  to  25  HP. 

We  are  now  in  a  position  to  review  the  divers  sorts  of  power 
transmission  that  have  been  discussed,  and  to  compare  them 
with  power  transmission  by  electricity. 
Without  going  deeply  into  details,  which  will  be  taken  up  in 


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GENERAL  CONDITIONS  OF  POWER  TRANSMISSION.    69 

due  course,  we  may  say  that  electrical  niachiuory  possesses  one 
advantage  to  an  unique  extent  —  high  efficiency  at  moderate 
loads.  Machinery  in  which  the  principal  losses  are  frictional 
is  subject  to  these  in  amount  nearly  independent  of  the  load; 
hence  the  efficiency  drops  rapidly  at  low  loads.  In  dynamos, 
motors,  and  transformers,  however,  the  principal  losses  de- 
crease rapidly  with  the  load,  so  that  within  a  wide  range  of 
load  the  efficiency  is  fairly  uniform.  Fig.  22  gives  the  efficiency 
curves  for  a  modem  dynamo,  motor,  and  transformer.     The 


100 

TRA 

1 

no 

/ 

/^ 

> 

,^ 

^^OVN^ 

hO 

z 
u 
o 

/ 

/ 

/I 

^0^0^ 

SO 

/ 

i 

1 

70 

/ 

> 

4 

\ 

i 

(       FULL 

LOAD 

Fio.  22. 

generator  curve  is  from  a  2(X)  KW  5()0-volt  direct-current 
machine,  the  motor  curve  from  a  smaller  machine  of  the  same 
type,  and  the  transformer  curve  from  a  standard  type  of  about 
30  kilowatts  capacity.  Jn  the  generator  curve  the  variation  of 
efficiency  from  half  load  to  full  load  is  less  than  2  per  cent, 
in  the  motor  only  2\  per  cent,  and  in  the  transformer  just 
1^  per  cent.  In  addition,  the  efficiency  of  all  three  at  full 
load  is  very  high.  Hence,  not  only  is  an  electrical  power 
transmission  of  great  efficiency  if  the  loss  in  the  line  be  moder- 
ate, but  this  efficiency  persists  for  a  wide  range  of  load.     As 


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60  ELECTRIC   TRANSMISSION  OF  POWER. 

in  hydraulic  and  pneumatic  transmission,  the  efficiency  of  the 
line  depends  on  its  dimensions;  so  that  by  increasing  the  weight 
of  copper  in  the  line,  the  loss  of  energy  may  be  decreased 
indefinitely.  And  since  the  loss  of  energy  in  the  line  dimin- 
ishes as  the  scjuare  of  the  current,  the  percentage  of  loss  at 
constant  voltage  diminishes  directly  with  the  load. 

Hence,  the  total  efficiency  may  be  constant  or  even  increase 
from  half  load  to  full  load,  even  with  a  quite  moderate  loss  in 
the  line.  In  pneumatic  and  hydraulic  transmission  this  con- 
dition may  occur,  but  only  with  large  loss  in  the  mains,  since 
the  efficiencies  of  the  generator  and  motor  parts  of  such  systems 
decrease  too  rapidly  to  be  compensated  by  the  gain  in  the 
main,  unless  its  efficiency  is  low  at  full  load.  Hence,  for 
ordinary  cases  of  distribution  in  which  the  average  load  is 
considerably  less  than  full  load,  often  only  J  to  ^  of  full  load, 
electric  transmission  has  a  very  material  advantage  over  all 
other  methods.  To  appreciate  this  we  need  only  to  run  over 
the  details  of  electrical  power  transmission  and  compare  the 
results  with  those  which  we  have  obtained  for  the  other 
methods  described. 

There  are  to  be  considered  in  electrical  power  transmission, 
as  in  transmission  of  every  sort,  two  somewhat  distinct  prob- 
lems : 

First,  the  transmission  of  energy  over  a  considerable  dis- 
tance and  its  utilization  in  one  or  a  few  large  units. 

Second,  the  distribution  of  power  to  a  large  number  of  small 
units  at  moderate  distances  from  the  centre  of  distribution. 
This  latter  case  may  sometimes  also  involve  the  transmission 
of  power  to  a  real  or  fictitious  centre  of  distribution.  This 
second  problem  is  the  commoner,  and,  while  not  so  sensational 
as  the  transmission  of  power  at  high  voltage  over  distances  of 
many  miles,  is  of  no  less  commercial  importance. 

We  have  all  along  been  considering,  in  treating  of  transmis- 
siori  of  power  by  ropes  and  by  hydraulic  and  pneumatic  engines, 
the  case  first  mentioned,  excepting  in  so  far  as  some  special 
distributions  have  been  referred  to.  We  have  already  the  data 
for  figuring  the  efficiency  of  an  electric  power  transmission 
•with  large  units.  In  cases  of  this  kind  the  distance  between 
the  generator  and  motor  is  likely  to  be  much  greater  than  in 


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GENERAL  CONDITIONS  OF  POWER  TRANSMISSION,    61 

the  case  of  distribution  to  small  motors  from  some  central 
point,  and  the  loss  in  the  line,  the  only  uncertain  figure  in  the 
transmission,  would  generally  range  from  5  to  10  per  cent. 
In  case  of  distributing  plants  intended  to  furnish  from  a  single 
point  small  units  of  power  over  a  moderate  distance,  it  is 
generally  found  that  losses  in  the  line  of  from  2  to  5  per  cent 
do  not  involve  excessive  cost  of  copper.  In  cases  where 
a.  distribution  is  coupled  with  the  transmission  of  power  to  the 
central  point,  the  loss  from  the  distant  generator  to  the  motors 
is  in  most  cases  from  10  to  15  per  cent. 

Taking  up  first  the  transmission  of  power  from  one  or  more 
large  generators  to  one  or  more  large  motors,  we  may  take 
safely  the  commercial  efficiency  of  the  generator  as  that  given 
by  the  curve.  Fig.  22,  and  that  of  the  motors  as  at  least  as 
good  as  that  given  for  a  motor  in  the  same  figure.  The  effi- 
ciency of  the  line  for  moderate  distances  may  be  taken  as 
95  per  cent.  It  should  be  noted  that  the  efficiencies  of  large 
alternating  generators  and  motors  do  not  differ  materially 
from  those  shown;  in  fact,  are  quite  certain  to  be  above  them. 
We  thus  have  for  the  efficiency  in  a  transmission  of  this  kind: 
94  X  95  X  93  =  84  per  cent.  This  is  largely  in  excess  of 
that  which  could  be  obtained  at  distances  of  say  a  couple  of 
miles  by  any  other  method  of  transmission. 

Even  more  extraordinary  is  the  efficiency  at  half  load  in 
this  ca.se,  which  is  92  x  97.5  x  91  =  81.6  per  cent.  It 
should  be  borne  in  mind  that  these  efficiencies  are  taken  from 
experiments  with  ordinary  machines,  and  the  efficiencies  are 
those  which  can  be  bettered  in  practice.  These  results  show 
the  great  advantage  to  be  derived  from  electrical  transmission 
when,  as  in  most  practical  cases,  full  load  is  seldom  reached. 
It  is  most  important  for  economical  operation  to  employ  a 
system  which  will  give  high  efficiency  at  low  loads,  and  it 
would  be  worth  while  so  to  do  even  if  the  efficiency  at  full 
load  were  not  particularly  good.  With  electrical  machinery, 
however,  there  is  no  such  disadvantage.  Even  at  one-fourth 
load  the  efficiency  of  the  electrical  system  still  remains  good. 
It  is  nearly  73  per  cent  on  the  a.ssumed  data.  The  efficiencies 
thus  given  are  from  the  shaft  of  the  generator  to  the  pulley 
of  the  motor  inclusive. 


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62 


ELECTRIC  TRANSMISSION  OF  POWER. 


In  the  case  of  distributed  motors  supplied  from  a  central 
point  not  very  distant  from  any  of  them,  the  efficiencies  of 
generator  and  line  remain  about  as  before,  but  the  motor 
efficiencies  for  the  sizes  most  often  employed  are  below  that 
just  given.  The  average  motor  efficiency  is  largely  dependent 
on  the  skill  with  which  the  units  are  distributed.  It  has  often 
been  proposed  to  drive  separate  machines  by  individual  motors, 
while  in  other  cases  comparatively  long  lines  of  shafting  are 
employed,  grouping  many  machines  into  a  dynamical  unit 
operated  by  a  motor.  To  secure  economy  it  is  desirable  on 
the  one  hand  to  use  fairly  large  motors  well  loaded,  while  on 
the  other  hand  the  losses  in  shafting  and  belting  must  be  kept 
down. 

The  larger  the  motors,  the  better  their  efficiency  at  all 
loads  and  the  less  the  average  cost  per  HP,  but  with  small 
motors  the  cost  and  inefficiency  of  shafts  and  belts  may  be 
in  large  measure  avoided.  The  most  economical  arrange- 
ment depends  entirely  upon  the  nature  of  the  load.  Much 
may  be  said  in  favor  of  individual  motors  for  each  machine, 
but  so  far  as  total  economy  is  concerned,  this  practice  is  best 
limited  to  a  few  cases  -  -  machines  demanding  several  HP  (say 
5  or  more)  to  operate  them,  machines  so  situated  as  to  neces- 
sitate much  loss  in  transmitting  power  to  them,  and  certain 
classes  of  portable  machines.  In  applying  electric  power 
to  workshops  already  in  operation,  the  group  system  will 
usuall}'^  give  the  best  results,  individual  motors  being  used 
only  for  such  machines  as  might  otherwise  cause  serious  loss 
of  power.  The  following  table  gives  the  average  full  load 
efficiencies  that  may  safely  be  expected  from  motors  of  various 
sizes,  irrespective  of  the  particular  type  employed. 


HP  of  motor  .   . 

1 
72 

8 

7S 

5 
81 

.1,. 

sa  j  80 

15 
86 

20 

87 

25 

88 

40 
90 

50 
90 

Per  cent  efficiency 

92 


These  are  commercial  efficiencies  reckoned  from  the  electri- 
cal input  to  the  mechanical  output  at  the  pulleys.  Below  5  HP 
the  efficiencies  fall  off  rapidly.     At  partial  loads  the  efficiencies 


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GENERAL  COSDITIOXS  OF  POWER   TRANSMISSIOX,    63 

are  somewhat  uncertain,  inasmuch  as  some  motors  are  designed 
so  as  to  give  their  maximum  efficiency  at  some  point  below 
full  load,  while  others  work  with  greater  and  greater  efficiency 
as  the  load  increases  until  heating  or  sparking  limits  the  out- 
put. The  former  sort  are  most  desirable  for  ordinary  work- 
shop use,  while  the  latter  are  well  suited  to  intermittent  work 
at  very  heavy  loads,  as  in  hoisting.  The  difference  in  the  two 
types  of  machine  is  very  material.  It  is  easily  possible  to 
procure  motors  that  will  not  vary  more  than  5  per  cent  in 
efficiency  from  full  load  to  half  load,  and  this  even  in  machines 
as  small  as  2  or  3  HP.  We  may  now  calculate  the  efficiency 
of  an  electric  distribution  with  motors  of  moderate  size  —  such 
a  case  as  might  come  from  the  electrical  equipment  of  large 
factories.  The  generator  efficiency  may  be  taken  as  before 
at  .94  and  that  of  the  line  at  .95,  while  the  motors  nuist  be 
laken  close  account  of  in  order  to  estimate  their  collective 
efficiency.  Assuming  the  sizes  of  motors  in  close  accordance 
with  those  in  several  existing  installations  of  similar  character, 
we  may  sum  them  up  about  as  follows: 

5  3  HP 

o  5  HP 

10  10  HP 

10  20  HP 

5  26  HP 

2  f)0  HP 

In  all  37  motors,  aggregating  565  HP.  The  mean  full  load 
efficiency  of  this  group  is  very  nearly  .N7.  The  efficiency  of 
the  system  is  then 

.94  X  .95  X  .87  =  77.6. 

This  result  requires  full  load  throughout  the  plant,  a  some- 
what unusual  condition  with  any  kind  of  distribution.  From 
the  data  already  given,  the  half  load  efficiency  should  be  about 

.92  X  .975  X  .82  -  .785. 

Between  the  limits  just  computed  should  lie  the  commercial 
efficiency  of  any  well-designed  motor  distril)ution  reckoned 
from  the  dynamo  pulley.  In  the  case  of  steam-driven  plants 
it  is  often  desirable  to  consider  the  indicated  HP  of  the  engine 
as  the  starting  point,  and  the  (juestion  immediately  arises  as 


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64 


ELECTRIC  TRANSMISSION  OF  POWER, 


to  the  commercial  efficiency  of  the  combination  of  d3mamo 
and  engine.  In  cases  where  high  efficiency  is  the  desideratum 
direct  coupling  is  usually  employed,  saving  thereby  the  loss 
of  power,  perhaps  5  per  cent,  produced  by  belting.  The 
losses  in  such  direct-coupled  units  vary  considerably  with  the 
size  and  type  of  both  machines.  Fig.  23  shows  the  efficiency 
of  two  such  combinations  at  various  loads.  Curve  A  is  from 
an  actual  test  of  the  combination;  curve  B  from  tests  of  an 
engine  and   dynamo  separately.     Each   unit  was  of  several 


xuo 
90 

m 

:r^ 

:== 

z 

~*7D 

y 

</ 

^^""^'^^^ 

.0^^^ 

b. 
bJ 

60 

/ 

^ 

/^ 

60 

A 

/ 

^ 

40 

/ 

1 

4 

k 

i 

H 

•4      FULL 

LOAD 

Fi«.  23. 

hundred  HP.     The  high  result  from  curve  A  is  mainly  due 
to  very  low  friction. 

These  curves  give  handy  data  for  computing  the  total  effi- 
ciency of  a  motor  plant  from  the  motor  pulleys  to  the  indicated 
horse-power  of  the  driving  engine.  Taking  the  combined 
engine  and  dynamo  efficiency  from  i4,and  assuming  the  same 
figures  as  before  on  motors  and  line,  we  have  at  full  load, 

.88  X  .95  X  .87  =  .727. 
And  from  the  same  data  at  half  load, 

.78  X  .975  X  .82  =  .651. 


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GENERAL  CONDITIONS  OP  POWER  TRANSMISSION.    65 

For  certain  computations,  as  in  case  of  figuring  out  a  com- 
plete installation,  the  above  efficiencies  are  convenient.  They 
show  that  in  very  many  instances  the  distribution  of  power  by 
electric  motors  is  very  much  more  economical  of  energy  than 
any  other  method  employed.  In  ordinary  manufacturing 
operations  power  is  generally  transmitted  to  the  working 
machines  through  the  medium  of  lines  of  shafting  of  greater 
or  less  length.  These  are  very  rarely  belted  directly  to  the 
machines,  but  transfer  power  to  them  through  one  or  more 
countershafts.  Often  the  direction  of  shafts  is  changed  by 
gearing  or  quarter  turn  belts,  and  even  when  the  power  is 
distributed  through  only  a  single  large  building  there  will  be 
found  more  often  than  not,  intervening  between  the  driving 
engine  and  the  driven  machine,  three  belts  and  two  lines  of 
shafting  of  consi  derable  length,  and  not  infrequently  still  other 
belts  and  shafts.  It  very  often  happens,  too,  that  to  keep  in 
operation  one  small  machine  in  a  distant  part  of  the  shop,  it  is 
necessary  to  drive  a  long  shaft  the  friction  of  which  consumes 
half  a  dozen  times  as  much  power  as  is  actually  needed  at  the 
machine.  The  constant  care  required  to  keep  long  lines  of 
shafting  in  operative  condition  is  an  irritating  and  costly  con- 
comitant. The  necessary  result  is  a  considerable  loss  of  power, 
which,  being  nearly  constant  in  amount,  is  very  severe  at 
partial  loads. 

Mowing  5  per  cent  loss  of  energy  for  each  transference 
of  power  by  belting,  a  figure  in  accordance  with  facts,  and  10 
per  cent  loss  for  each  long  line  shaft  driven,  it  Is  sufficiently 
evident  that  from  20  to  25  per  cent  of  the  brake-horse-power 
delivered  by  the  engine  must  be  consumed  even  under  very 
favorable  circumstances  by  the  belting  and  shafting  at  full 
load.  This  means  an  efficiency  at  half-load  of  from  50  to  60 
per  cent  only,  and  at  le&ser  loads  a  very  low  efficiency  indeed. 

The  large  number  of  careful  experiments  carried  out  on 
shafting  in  different  kinds  of  workshops,  and  under  various 
conditions,  shows  that  only  imder  very  exceptional  circum- 
stances LS  the  loss  of  power  by  shafting  between  the  engine 
and  the  driven  machines  as  low  as  25  per  cent.  Far  more 
often  it  is  from  30  to  50  per  cent,  and  sometimes  as  high  as 
75  or  80  per  cent.     The  figures,  which  have  been  well  estab- 


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66  ELECTRIC  TRANSMISSION  OF  POWER. 

lished,  regarding  the  efficiency  of  the  transmission  of  power 
by  motors,  show  that  at  full  load  it  is  comparatively  easy 
to  exceed  75  per  cent  efficiency;  thus  more  than  equalling  the 
very  best  results  that  can  be  obtained  with  shafting.  At  half- 
load  and  below,  the  advantage  of  the  electric  transmission  be- 
comes enormous,  even  supposing  shafting  to  be  at  its  very 
best. 

Compared  with  ordinary  transmission  by  shafting,  the  motor 
system  is  incomparably  superior  at  all  loads,  so  that  it  may 
easily  happen  that  a  given  amount  of  work  can  be  accomplished 
through  the  medium  of  a  motor  plant  with  one-half  the  steam 
power  required  for  the  delivery  of  the  same  power  through 
shafts  and  belts.  Such  results  as  this  have  actually  been 
obtained  in  practice.  It  is,  therefore,  safe  to  conclude  that 
the  distribution  of  power  by  motors  is,  under  any  ordinary 
commercial  conditions,  at  least  as  efficient  as  the  very  best 
distribution  of  power  by  shafting  at  full  load,  and  much  more 
efficient  at  low  loads.  Under  working  conditions  in  almost  all 
sorts  of  manufacturing  establishments,  light  loads  are  the  rule 
and  full  loads  the  rare  exception;  consequently  the  results  of 
displacing  shafting  by  motor  service  have,  as  a  rule,  been 
exceedingly  satisfactory  in  point  of  efficiency,  and  the  lessened 
operating  expense  more  than  offsets  the  extra  cost  of  installa- 
tion. 

In  one  early  three-phase  plant,  that  of  Escher,  Wyss  &  Co. 
at  Winterthur,  Switzerland,  300  HP  in  32  motors  worked  from 
a  12-mile  transmission  line  displaced  far  greater  capacity  in 
steam  engines,  and  similar  results  on  a  smaller  scale  are  not 
uncommon. 

To  add  force  to  this  comparison  between  the  efficiency  of 
shafting  and  of  motors,  the  following  results  from  electrical 
distribution  plants  already  installed  may  be  pertinent.  A 
typical  example  of  the  sort  is  from  a  plant  installed  some  years 
since  in  a  fire-arms  factory  at  Herstal,  Belgium.  There  were 
there  installed  17  motors  of  an  aggregate  capacity  of  305  HP, 
driven  by  a  300-KW  generator  direct-coupled  to  a  500-HP 
compound -condensing  engine.  The  efficiency  guaranteed  from 
the  shaft  of  dynamo  to  the  pulleys  of  the  motors  is  77  per  cent. 
Since  its  first  installati(m,  the  plant  has  been  increased  by  the 


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GENERAL  CONDITIONS  OF  POWER  TRANSMISSION.    67 

addition  of  a  second  direct-coupled  dynamo  and  the  total  horse- 
power of  motors  is  428.  A  second  notable  installation  of 
motors  in  the  same  vicinity  is  at  the  metallurgical  works  of 
La  Soci^t^  de  la  Vielle-Montagne,  consisting  of  a  375-KW  500- 
volt  dynamo  direct  .driven  at  a  speed  of  80  revolutions  per 
minute  by  a  600-HF  compound-condensing  engine.  The  plant 
consists  of  37  motors  with  an  aggregate  HP  of  329.  The  full 
load  efficiency  of  the  plant  from  dynamo  shaft  to  motor  pulley 
is  76  per  cent.  The  loss  in  the  lines,  both  in  this  case  and 
in  the  preceding,  is  very  small,  only  2  per  cent.  They  are 
both  typical  cases  of  transmission  to  motors  driving  groups 
of  machines,  and  in  spite  of  rather  low  dynamo  efficiencies 
at  full  load,  these  being  in  each  case  90  per  cent,  the  results 
obtained  are  in  close  accordance  with  those  already  stated  as 
appropriate  to  similar  cases.  As  an  example  of  work  under 
more  favorable  conditions,  the  early  three-phase  power  plant 
at  Columbia,  S.  C,  may  be  instanced. 

The  problem  here  undertaken  was  to  drive  a  very  large  cot- 
ton mill,  utilizing  for  the  purpose  a  water-power  about  800  feet 
distant.  Two  500-KW  dynamos  direct-coupled  at  a  speed  of 
108  turns  per  minute  deliver  current  at  550  volts  to  an  under- 
ground line  connecting  the  power  station  with  the  mills. 
The  motors  are  suspended  from  the  ceiling,  and  each  drives 
several  short  countershafts.  The  motors  are  wound  for  the 
generator  voltage  without  transformers,  and  are  of  a  uniform 
size,  65  HP  each.  The  commercial  efficiency  of  this  plant, 
taken  as  a  whole  from  the  shaft  of  the  dynamo  to  the  pulleys 
of  the  motors,  is  not  less  than  82  per  cent  at  full  load.  This 
good  result  is  due  to  the  use  of  large  motors,  and  to  the  small 
line  loss  of  2  per  cent  as  in  the  preceding  foreign  examples. 
These  results  are  thoroughly  typical,  and  can  regularly  be 
repeated  in  practice.  In  general  a  net  efficiency  of  80  to  85 
per  cent  can  be  counted  upon  in  plants  of  the  approximate 
size  of  those  here  mentioned,  assuming  the  apparatus  now 
commercially  standard.  Even  smaller  plants  can  be  counted 
on  to  give  nearly  or  quite  as  good  results,  since  the  differ- 
ence in  efficiency,  supposing  motors  of  the  same  size  to  be 
used,  between  a  d3mamo  of  100  KW  and  one  of  400  or  500  KW 
is  hardly  more  than  1  |>er  cent  at  full  load,  supposing  machines 


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68  ELECTRIC  TRANSMISSION  OF  POWER. 

of  the  same  general  design  to  be  employed,  nor  is  there  any 
substantial  difference  in  efficiency  between  plants  employing 
direct  current  and  those  using  polyphase  apparatus,  as  may 
be  judged  from  the  figures  just  given. 

We  are  now  in  position  intelligently  to  compare  the  trans- 
mission and  distribution  of  power  by  electric  means  with  the 
other  methods  which  have  sometimes  been  employed. 

All  comparisons  between  methods  of  transmitting  power 
have  to  be  based  in  a  measure  on  their  relative  efficiency. 
Now,  in  every  such  method  there  are  three  essential  factors: 
1st,  the  generating  mechanism,  which  receives  power  direct 
from  the  prime  mover  and  in  conjunction  with  which  it  is 
considered;  2d,  the  transmitting  mechanism,  which  may  be  an 
electric  line,  a  pipe  line,  ropes,  or  belts;  and  3d,  the  motor 
part  of  the  transmission,  which  receives  power  from  the  trans- 
mitting mechanism  and  delivers  it  for  use.  For  a  given 
capacity  of  the  generating  and  receiving  mechanisms,  the 
efficiency  of  each  at  all  loads  is  determined  within  fairly  close 
limits.  The  transmitting  mechanism,  however,  is  not  so 
closely  determined,  save  in  the  case  of  the  rope  drive. 

Electric,  pneumatic  and  hydraulic  transmission  lines  are  all 
subject  to  the  general  principle  that  the  loss  in  transmission 
can  be  made  indefinitely  small  by  an  indefinitely  large  expen- 
diture of  capital,  enormous  cross-section  in  the  one  case,  or 
huge  pipe  lines  in  the  others.  The  efficiency  of  these  methods 
is,  therefore,  a  fluctuating  quantity  depending  on  that  loss  in 
the  transmitting  mechanism  which  may  be  desirable  from  an 
engineering  or  economical  standpoint.  In  making  compar- 
isons between  these  methods,  there  is  a  wide  opportunity  for 
error  unless  some  common  basis  of  comparison  is  predeter- 
mined. In  the  next  case  any  such  comparison  must  differ 
widely  in  its  results  according  to  the  character  of  the  power 
distribution  which  is  to  be  attempted.  We  have  already  seen 
that  with  the  rope  drive,  distribution  is  very  difficult,  while 
with  electric  and  pneumatic  systems  it  is  comparatively 
easy. 

A  general  valuation  of  the  commercial  possibilities  of  these 
divers  matters  is,  therefore,  hard  to  make  except  in  a  general 
way.     We  can,  however,  by  assuming  a  given  transmission  of 


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GENERAL  CONDITIONS  OF  POWER   TRANSMISSION.    69 

given  magnitude  and  character,  and  further  assuming  such 
loss  in  the  transmitting  mechanism  as  might  reasonably  be 
expected  in  practice,  arrive  at  a  reasonably  accurate  conclu- 
sion for  the  case  considered.  As  a  very  simple  example  of 
power  transmission,  let  us  take  the  delivery  of  power  over  a 
distance  of  two  miles,  the  delivery  being  in  one  unit  or,  at 
most,  two  units.  We  will  assume  the  same  indicated  HP  fur- 
nished at  the  generating  end  of  the  line  in  each  case,  of  which 
as  much  as  possible  is  to  be  delivered  at  the  receiving  station, 
the  losses  in  transmissicm  being  taken  as  10  per  cent  of  the 
power  delivered  to  the  line;  this  is  to  cover  all  losses  of 
energy  by  resistance  and  leakage  on  the  electrical  line  or 
loss  of  pressure  and  resulting  expenditure  of  energy,  leakage, 
friction,  and  all  other  sources  of  loss  in  the  other  cases. 

As  the  same  indicated  power  is  generated  in  each  case,  we 
will  suppose  a  modem  plant  with  compound  condensing  en- 
gines costing  complete  with  buildings  $50  per  HP.  We  will 
further  assume  that  each  indicated  horse-power  per  working 
year  of  3,000  hours  will  cost  $18;  this  covering  all  expenses 
except  those  chargeable  to  interest  and  depreciation.  For 
this  simple  case  we  have  the  following  costs  of  initial  plant 
and  of  operation  per  mechanical  horse-power  delivered  from 
the  motor,  full  load  only  })eing  considered.  The  four  meth- 
ods considered  are  rope  driving,  pneumatic,  pneumatic  with 
reheating  apparatus  at  the  motors,  and  electrical.  The  prices 
are  from  close  estimates  of  the  cost  in  each  case.  The  dyna- 
mos are  supposed  to.  be  direct-coupled.  The  compressors  tr) 
be  direct-acting,  two-stage  compressors.  The  steam  cylinders 
Corliss  compound-condensing  type.  The  air-pressure  assumed 
is  60  lbs.  above  atmospheric  pressure.  The  electric  voltage 
3,000.  The  rope  speed  about  one  mile  per  minute.  Interest 
and  depreciation  are  taken  at  10  per  cent  of  the  total  cost  of 
the  plant,  save  in  the  case  of  the  rope  drive,  where  an  addi- 
tional charge  for  renewal  of  cable  is  made  on  the  supposition 
that  the  cable  will  last  somewhere  from  18  months  to  2  years, 
which  is  fully  as  favorable  a  result  as  can  fairly  be  expected. 
The  following  are  the  comparative  estimates: 


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70  ELECTRIC   TRANSMISSION   OF  POWER. 

Rope,  EFFiciENrY  67  per  cent. 

COST. 

Steam  plant $50,000 

Pulley  stations 26,000 

Cables,  steel 17,000 

ToUlcost 192,000 

OPERATING    EXPENSE. 

1,000  l.HP  at  ai8 118,000 

Interest  and  depreciation  on  plant,  at  10  per  cent  .        .        .  7,500 

Depreciation  of  cable 8,000 

133,500 
Net  HP  produced,  672. 
Cost  per  HP-year,  $49. 

Pneumatic,  Efficiency  64  per  cent. 

COST. 

Steam  plant,  excluding  engines $35,000 

Compressors    . 17,000 

Air  mains  laid,  12  inches 18,000 

Air  motors 12,000 

Total  cost $82,000 

operating  expense. 

1,000  I.HP  at  $18 $18,000 

Interest  and  depreciation,  at  10  per  cent 8,200 

$26,000 
Net  HP  delivered,  640. 
Cost  per  HP-year,  $48. 

Air  Reheated,  Apparent  Efficiency  65  per  cent. 

COST. 

Steam  plant,  excluding  engines $35,000 

Compressors 17,000 

Air  mains  laid 18,000 

Air  motors  and  rchealers  with  chimney,  etc.          ....  14,000 

Total  cost $84,000 

operating  expense. 

1,000  I.HP  at  $18   .        .• $18,000 

Interest  and  depreciation,  at  10  per  cent 8,400 

Coal  and  labor  for  reheating 1,500 

$27,900 
Net  HP  delivered,  650. 
Cost  per  HP-year,  $43. 


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GENERAL  CONDITIONS  OF  POWER   TRANSMISSION.    71 
Electric,  Efficiency  73  per  cent. 

COST. 

Steam  plant 860,000 

Dynamos 18,000 

Line 3,000 

Motors 13,000 

Total  cost $84,000 

OPERATING    EXPENSE. 

l,000I.HPat$18 $18,000 

Interest  and  depreciation,  at  10  per  cent 8,400 

Electrician 1,500 

$27,900 
Net  HP,  730. 
Cost  per  HP-year,  $38. 

It  appears  at  once  that  the  rope  drive  is  beyotul  the  range  of 
its  efficient  use.  Its  first  cost  is  greater  than  that  of  either  of 
the  other  methods,  and  the  expense  is  carried  to  a  very  high 
figure  by  the  item  of  depreciation  on  the  cables,  which  cannot 
be  avoided;  hence  in  spite  of  a  high  efficiency,  the  cost  per 
HP  year  delivered  rises  to  $49.  We  may  next  consider  the 
schedule  of  cost  for  the  pneumatic  system.  In  this  case  the 
most  formidable  item  is  the  cost  of  the  air-mains,  which  should 
be  at  least  12  inches  in  diameter.  Nevertheless,  the  total  initial 
cost  is  the  lowest  of  the  four.  The  operating  expense  is  also 
the  lowest,  but  the  very  low  efficiency  of  the  pneumatic  system 
without  reheating  raises  the  cost  per  HP  delivered  to  a  very 
considerable  amount  —  almost  as  much  as  in  the  case  of  the 
rope  drive.  Reheating  would  almost  always  be  used  in  con- 
nection with  a  plant  of  this  size,  and  with  reheating  the  result 
is  much  more  favorable.  The  initial  expenditure  is  somewhat 
increased  by  the  addition  of  the  reheaters,  piping  and  chinniey. 
The  operating  expense  is  also  slightly  increased  by  the  coal 
necessary  for  reheating,  taken  at  J  of  a  pound  per  HP  per  hour, 
and  the  small  amount  of  additional  labor  involved  in  caring 
for  the  reheaters,  disposing  of  the  ashes,  and  looking  after 
the  reheating  plant  generally.  The  apparent  efficiency  in 
this  Case  is  very  excellent,  65  per  cent  being  reasonably  attain- 
able, and  the  cost  per  HP  year  falls  to  $43,  shov/ing  conclu- 


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72  ELECTRIC   TRANSMISSION  OF  POWER. 

sively  enough  the  advantage  of  reheating;  at  least,  where 
the  units  are  so  large  that  the  presence  of  a  reheater  is  not 
a  practical  nuisance. 

Finally,  we  come  to  the  electric  power  transmission.  In 
this  case  the  most  striking  feature  is  the  low  cost  of  the  line, 
supposed  here  to  be  overhead.  It  may  be  noted,  however, 
that  an  underground  line,  consisting  of  cable  laid  in  conduit, 
still  leaves  the  cost  per  HP  year  lower  than  that  of  any  of 
the  other  methods.  Operating  expense  is  fairly  increased 
by  the  addition  of  an  electrician  to  the  cost  of  the  indicated 
horse-power,  interest,  and  depreciation.  The  total  first  cost 
ife  practically  the  same  as  that  of  air  with  reheater,  as  is  also 
the  operating  expense.  Tlie  added  efficiency,  however,  brings 
the  cost  per  HP  year  to  $38;  decidedly  the  lowest  of  the  four 
cases  considered.  It  may  be  thought  that  difference  of  loss 
in  transmission  might  possibly  alter  the  relation  of  the  electric 
plant  to  the  air-plant  with  reheaters,  but  an  added  efficiency 
of  line  would  in  either  case  be  accompanied  by  added  expen- 
diture of  not  very  different  amounts  in  the  two  cases,  and  the 
efficiency  of  the  electric  plant  would  always  be  enough  higher 
than  that  of  the  air-plant  to  give  it  the  advantage  in  net  cost 
per  HP,  however  the  two  plants  might  be  arranged.  We  thus 
find  that  at  a  distance  of  two  miles  the  electric  transmission 
has  a  material  advantage,  air  with  reheaters,  air  without 
reheaters,  and  rope  drive  following  it  in  the  order  named. 
The  pneumatic  method  would  at  the  distance  of  one  mile, 
as  may  readily  be  computed,  take  about  the  same  relative 
position  as  before,  since  the  efficiency  maintains  approxi- 
mately the  same  relation  to  the  others. 

The  pneumatic  plant  gains  in  first  cost  at  this  lesser  dis- 
tance, not  enough,  however,  to  alter  the  final  result.  At 
half  a  mile  distance,  the  rope  drive  will  be  found  to  be  the 
cheapest  in  fii-st  cost,  and  also,  through  its  enormous  efficiency, 
to  be  a  little  the  cheapest  per  HP  delivered,  in  spite  of  the 
large  depreciation  in  the  cables,  while  the  electric  and  pneu- 
matic systems  would  be  very  close  together,  the  electric, 
however,  still  retaining  a  slight  advantage  due  to  its  greater 
efficiency.  Neither  can,  in  point  of  absolute  cost  of  power 
delivered,  compete  with  the  rope  drive  at  this  distance  for 


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GENERAL  CONDITIONS  OF  POWER   TRANSMISSION.    73 

this  simple  transmission  at  full  load,  although  both  would 
surpavss  it  were  there  any  considerable  distribution  of  the 
powder.  Figures  that  have  heretofore  been  given  on  the 
relative  cost  and  efficiency  of  such  transmissions  have  as  a 
rule  been  in  error  in  two  very  essential  particulars:  first,  the 
efficiencies  of  the  electrical  system  have  been  greatly  under- 
estimated owing  to  the  poor  machines  with  which  the  first 
experiments  were  made;  second,  the  commercial  advantage  of 
reheating  in  the  pneumatic  transmission  has  not  generally  been 
given  its  proper  weight.  It  is,  as  has  been  already  stated,  not 
a  method  of  increasing  the  efficiency,  but  of  increasing  the 
power  delivered  by  addition  of  energy  at  the  receiving  end  of 
the  line  under  very  favorable  conditions.  The  figures  just 
given  are  believed  to  be  as  nearly  exact  as  roughly  assumed 
conditions  permit. 

One  modification  in  the  electric  transmission  should  here 
be  noted.  The  recent  introduction  of  the  steam  turbine  has 
rendered  it  possible  to  lower  the  cost  of  the  generating  plant 
very  materially,  while  retaining  a  cost  of  power  at  the  prime 
mover  not  in  excess  of  that  here  given.  The  generating  unit 
also  comes  in  for  some  reduction  in  the  combined  frictional 
loss,  so  that  the  final  cost  per  HP  year  would  on  the  basis  here 
taken  probably  fall  to  $35  or  $36,  giving  the  electrical  system 
a  still  greater  advantage.  This  statement  does  not  mean 
broadly  that  a  turbo-generator  can  regularly  deliver  power 
six  or  eight  per  cent  cheaper  than  an  ordinary  generating 
set,  but  merely  that  it  would  probably  do  so  under  the  cir- 
cumstances here  assumed. 

All  these  estimates  are  subject  to  change  of  prices  from 
year  to  year,  but  no  changes  are  likely  to  be  sufficient  to  alter 
the  relative  position  of  the  methods  compared  as  regards 
cost.  It  is  not  a  difficult  matter  to  construct  a  set  of  estimates 
arranged  to  favor  any  given  method.  In  the  long  run,  what- 
ever minor  variations  may  appear  in  the  items  here  given, 
the  totals  will  be  found  to  scale  up  or  down  in  about  the  same 
ratios. 

At  less  than  full  load  and  hence  under  variable  loads,  the  elec- 
tric system  enjoys  the  unique  advantage  of  having  the  losses 
of  energy  in  every  part  of  the  system  decrease  as  the  load 


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74  ELECTRIC   TRANSMISSION  OF  POWER. 

decreases,  while  in  rope  driving  all  the  losses  are  practically 
constant,  and  in  the  hydraulic  and  pneumatic  systems  all 
arc  nearly  constant  save  that  in  the  pipe-line. 

Hence,  under  low  and  varying  loads,  electric  transmission  has 
a  great  additional  advantage.  Since  in  distributions  of  power 
employing  a  considerable  number  of  motors  light  load  on  the 
motors  is  the  invariable  rule,  as  soon  as  we  depart  from  the 
very  simple  case  discussed  the  electrical  system  gains  in  rela- 
tive economy  at  every  departure.  These  more  general  cases 
have  already  been  described,  and  gathering  the  results  we 
may  construct  the  following  table,  showing  the  efficiency  of 
each  svstem  under  full  and  half-loads: 


System.  Full  l.osid.  Half  Load. 


Wire  rope .  67  46 

Hydraulic  high  pressure 53  46 

Hydraulic  low  pressure 6()  5() 

Pneumatic ^ 60  40 

Pneumatic  reheated  (virtual  efficiency)     .  66  60 

Electric 73  66 


The  efficiencies  in  the  electric  system  as  here  given  are 
lower  than  would  be  reached  practically  in  large  plants.  The 
present  practice  of  using  generators  and  motors  woimd  for 
pressures  up  to  10,000  or  12,000  volts  makes  a  most  material 
diiTerence  in  the  matter  of  efficiency.  For  a  well-designed 
transmission  of  a  few  miles  in  units  of  say  500  KW  and  up- 
wards, one  may  fairly  expect  to  get  at  full  load  as  much  as  94 
per  cent  from  generator  and  motor,  and  i)erha})s  98  per  cent 
from  the  line,  givhig  a  total  efficiency  of  transmission  of 

.94  X  .94  X  .98  =  .866 

at  full  load  and  of  nearly  .85  at  half-load  or  say  79  and  72  per 
cent  respectively  when  reckoning  efficiency  from  the  indicated 
HP  of  the  engine  as  in  the  foregoing  comparisons.  This  means 
far  higher  efficiency  than  can  be  obtained  by  any  other  method 
at  any  but  the  shortest  distances. 

All  the  figures  must  be  taken  as  approximate.  They  are 
under  conditions  fairly  comparable  except  in  case  of  the  low- 


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GENERAL  CONDITIONS  OF  POWER   TRANSMISSION.    75 

pressure  hydraulic  system,  in  which  the  large  proix)rti()n  of 
loss  due  to  pipe-friction  operates  to  hold  up  the  half-load  effi- 
ciency to  an  abnormal  degree.  With  the  ordinary  proportion 
of  small  motors  this  half-load  efficiency  would  be  nearer  40 
than  50  per  cent.  The  electric  system  is  easily  the  most 
efficient  at  any  and  all  loads.  Of  the  others,  wire-rope  trans- 
mission, if  the  distributed  units  are  fairly  large,  holds  the 
second  place  for  short  distances,  and  the  pneumatic  system 
with  energy  added  at  the  motors  by  reheating,  at  moderate 
and  long  distances.  Without  reheating  it  occupies  the  last 
place  in  order  of  efficiency,  although  even  so,  it  is,  next  to 
electricity,  the  most  convenient  method  of  distributing  power. 

In  fact,  electricity  and  compressed  air  are  the  only  two 
systems  available  for  the  general  distribution  of  energy.  The 
latter  is,  save  for  a  single  system  in  Paris,  used  only  on  a  small 
scale,  and  in  this  country  hardly  at  all  save  in  mining.  Of 
course,  the  very  largest  power  stations  are  those  belonging  to 
electric  railway  systems  in  the  largest  American  cities.  Several 
of  these  exceed  50,000  HP  in  generator  capacity  and  frequently 
in  actual  output,  notably  the  systems  in  Boston,  Brooklyn, 
and  Philadelphia.  Recent  advances  in  electrical  engineering, 
particularly  the  effective  utilization  of  alternating  currents, 
have  greatly  cheapened  the  distribution  of  electrical  energy, 
and  other  systems  are  now  seldom  installed  for  ordinary  pur- 
poses. A  few  pneumatic  and  hydraulic  plants  will  continue 
to  be  used,  owing  to  the  large  capital  already  invested  in  them, 
but  new  work  is,  and  in  the  nature  of  things  must  be,  almost 
exclusively  electrical.  As  the  transmission  of  power  from 
great  distances  becomes  more  common  and  the  radii  of  dis- 
tribution themselves  increase,  the  electrical  methods  gain  more 
and  more  in  relative  value,  and  all  others  become  more  ineffi- 
cient and  impracticable. 

We  have  now  discussed  in  some  detail  the  sources  of  natural 
energy  which  are  available  for  human  use,  and  the  most  promi- 
nent of  the  systems  employed  for  their  utilization.  We  have 
found  that  for  practical  purposes  steam-power  and  water- 
power  must  at  present  be  used  to  the  virtual  exclusion  of  all 
others,  the  former  perhaps  less  than  the  latter  save  for  dis- 
tribution of  power  over  short  distances. 


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76  ELECTRIC   TRANSMISSION  OF  POWER. 

Of  the  methods  of  distribution  we  have  found  all  save  com- 
pressed air  and  electricit}'-  very  limited  in  their  application, 
the  hydraulic  systems  to  special  classes  of  work  under  favorable 
topographical  conditions,  and  rope  transmission  to  extremely 
short  distances  and  small  numbers  of  power  units  delivered. 
Both  are  noticably  inefficient.  The  pneumatic  system  is  very 
general  in  its  applicability,  but  of  very  low  intrinsic  efficiency. 
When  used  in  connection  with  reheating  apparatus  it  requires 
additional  care,  and  the  motors,  like  steam-engines,  are  heavy 
and  inconvenient.  The  electric  system  on  the  other  hand  em- 
ploys motors  which  are  compact  and  far  more  efficient  than 
any  other  type  of  machine  for  delivering  mechanical  power, 
run  practically  without  attention,  and  can  be  placed  in  any  sit- 
uation or  position  that  is  convenient.  Furthermore,  in  average 
working  efficiency  the  electric  system  is  10  to  15  per  cent 
higher  than  any  other  yet  devised,  so  that  it  is  more  economical 
in  use  at  nearly  all  distances  and  under  nearly  all  conditions. 
Finally,  it  unites  with  power  distribution  the  ability  to  furnish 
light  and  heat,  thus  gaining  an  immense  commercial  advan- 
tage. This  advantage  is  shared  only  by  gas  transmission, 
which  up  to  the  present  time  remains  of  doubtful  value  on 
account  of  the  cost  of  the  motors,  their  imperfect  regulation, 
and  their  inefficiency  at  moderate  loads.  Gas  transmission, 
however,  is  likely  to  grow  in  importance,  owing  to  the  great 
improvements  in  small  gas-engines  stimulated  by  the  rapid 
development  of  the  automobile.  Having  now  overlooked 
the  advantages  of  electrical  power,  it  is  proper  to  pass  to  the 
details  of  the  methods  employed  for  its  utilization,  and  thence 
to  the  general  problem  of  its  economical  generation,  trans- 
mission and  distribution. 


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CHAPTER  ITT. 

POWER    TRANSMISSION    BY    CONTINXTOUS    TFRRENTS. 

Up  to  the  present  time  the  larger  part  of  electrical  power 
transmission  has  been  done  by  continuous  currents.  All  the 
earlier  plants  were  of  this  type,  and  even  now,  when  trans- 
mission by  alternating  currents,  polyphase  and  other,  is  gen- 
erally used,  the  older  type  of  apparatus  is  still  being  installed 
on  an  extensive  scale,  and  on  account  of  the  large  number  of 
plants  now  in  operation,  even  if  for  no  other  reason,  wdll  prob- 
ably remain  in  use  for  a  long  time  to  come.  New  power 
transmission  plants,  both  here  and  abroad,  are,  save  for  rare 
exceptions,  for  alternating  currents,  and  in  many  cases  this 
practice  is  almost  absolutely  necessary,  but  there  still  remain 
many  cases  wherein  the  conditions  are  as  well  met  in  the  old- 
fashioned  way. 

Chief  among  these  may  be  mentioned  electric  railway  work, 
which  in  America  alone  certainly  requires  more  than  a  full 
million  horse-power  in  generators  and  motors.  Certain  diffi- 
cult work  at  variable  speed  and  load,  and  many  simple  trans- 
missions over  short  distances,  are  at  present  best  handled  by 
continuous  current  machinery.  As  alternating  practice  ad- 
vances, many,  perhaps  all,  of  these  special  cases  will  be  elimi- 
nated, but  we  are  dealing  with  the  art  of  power  transmission  as 
it  exists  to-day,  and  hence  continuous  current  working  deserves 
consideration. 

The  broad  principle  of  the  continuous  current  generator  has 
already  been  explained,  but  its  modifications  in  actual  work 
are  important  and  worthy  of  special  investigation.  Tn  a  gen- 
eral way,  continuous  currents  are  almost  always  obtained  by 
commuting  the  current  obtained  from  a  machine  which  would 
naturally  deliver  alternating  currents.  This  process  is,  how- 
ever, by  no  means  a.s  simple  as  I"ig.  9  would  suggest.  With  a 
two-part  commutator  the  resulting  current,  although  unidirec- 
tional, would  necessarily  be  very  irregular,  owing  to  the  fact 

77 


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78 


KLECTRIC  TRANSMISSION  OP  POWER, 


that  the  total  current  drops  to  zero  at  the  moment  of  com- 
mutation. Such  a  current  is  ill  fitted  for  many  purposes,  and 
the  commutator  would  be  rapidly  destroyed  by  sparking  if  the 
machine  were  of  any  practical  size. 

To  avoid  these  difficulties,  the  number  of  coils  on  the  arma- 
ture is  increased,  and  they  are  so  interconnected  that,  while 
each  coil  has  its  connection  to  the  outside  current  reversed  as 
before,  when  its  electromotive  force  is  zero,  the  other  coils  in 
which  the  E.  M.  F.  still  remains  in  the  right  direction  continue 
in  circuit  imchanged.  In  this  way  the  E.  M.  F.  at  the  brush 
is  the  sum  of  the  E.  M.  F.'s  of  a  number  of  coils,  each  of  which 
is  reversed  at  the  proper  moment.  The  number  of  commutator 
segments  is  increased  proportionally  to  the  number  of  coils, 


Fia.  24. 

and  the  commutator  thus  becomes  a  comparatively  complicated 
structure.  The  result,  however,  is  that  the  total  E.  M.  F.  of 
the  armature  cannot  vary  by  more  than  the  variation  due  to  a 
single  coil.  The  nature  of  this  modification  is  shown  in  Fig. 
24,  which  shows  a  four-part  commutator  connected  to  a  four- 
coil  drum  armature. 

An  eight-part  winding  of  modern  type  is  shown  in  Fig.  25. 
Tracing  out  the  currents  in  this  will  give  a  clear  idea  botli  of 
a  typical  winding  and  of  the  process  of  commutation. 

In  commercial  machines  the  number  of  individual  coils  and 
of  commutator  segments  often  exceeds  100,  but  the  principle 
of  the  winding  is  the  same.  Nearly  all  the  early  dynamos  had 
several  turns  of  wire  per  coil,  as  in  Fig.  24,  but  at  present,  in 
most  large  machines,  one  turn    constitutes  a  complete   coil. 


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TRANSMISSlOyi  liY  CONTINUOUS  CURRENTS. 


79 


This  subdivision  is  to  avoid  sparking  at  the  commutator, 
which  becomes  destructive  if  the  current  be  large  and  the 
E.  M.  F.  per  commutator  segment  more  than  a  few  volts. 

If  each  coil  generates  a  considerable  voltage,  there  is  even 
under  the  best  conditions  of  commutation  a  strong  tendency 
for  sparks  to  follow  the  brush  across  the  insulation  between 
segments,  or  even  to  jump  across  this  insulation  elsewhere. 
As  this  goes  from  bad  to  worse,  and  rapidly  ruins  the  commu- 
tator, every  precaution  has  to  be  taken  against  such  a  con- 
tingency. The  E.  M.  F.  generated  by  each  coil  is  kept  low 
by  subdividing  the  winding,  and  in  large  machines  it  is  the 


FlQ.  26. 

rule  that  the  E.  M.  F.  of  a  single  loop  is  quite  all  that  can 
safely  be  allotted  to  a  single  commutator  segment. 

Present  good  practice  indicates  that,  in  generators  for  light- 
ing, up  to  100  or  150  volts  the  voltage  between  brushes  should 
be  subdivided  so  that  it  shall  not  exceed  3  or  4  volts  for 
each  segment  between  the  brushes.  For  500  or  600  volt 
machines  it  should  not  ordinarily  exceed  8  or  10  volts,  while 
for  dynamos  of  moderate  output  and  even  higher  voltage  it 
may  rise  to  20  volts  or  more. 

The  reason  for  these  differont  figures  is  that  the  destructive- 


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80 


ELECTRIC  TRANSMISSION  OF  POWER. 


ness  of  the  spark  depends  on  the  amount  of  current  which  is 
liable  to  be  involved.  On  a  low  voltage  commutator  intended 
for  heavy  currents,  even  very  moderate  sparking  may  gnaw 
the  segments  seriously,  while  the  spark  of  an  arc  machine,  in 
spite  of  its  venomous  appearance,  may  do  very  little  harm,  as 
the  maximum  current  in  the  whole  bar  will  not  exceed  8  or  10 
amperes.  Consequently  the  voltage  per  bar  in  such  cases  is 
sometimes  50  or  more,  while  in  very  large  incandescent  ma- 
chines, and  in  those  designed  for  electrolytic  purposes,  the 
E.  M.  F.  per  bar  is  often  less  than  2  volts  or  even  below  1  volt. 
Windings  like  those  of  Figs.  24  and  25  are  of  the  so-called 


Flo.  2G. 

dram  type,  in  which  each  convolution  extends  around  the  whole 
body  of  the  armature,  either  diametrically  or  nearly  so. 
Another  sort  of  armature  winding  frequently  used,  although 
less  now  than  formerly,  is  the  Gramme,  so  called  from  its 
inventor.  Here  the  iron  body  of  the  armature  is,  instead 
of  being  cylindrical,  in  the  form  of  a  massive  ring  of  rect- 
angular cross-section.  The  windings  are  looped  through  and 
around  this  ring,  fitting  it  firmly  and  closely.  Fig.  26,  which 
shows  in  diagram  a  winding  in  ten  sections,  furnishes  a  good 
example  of  the  Gramme  construction.  There  may  be  one  or 
several  turns  per  coil,  as  in  drum  windings.  These  two  gen- 
eral types  of  windings  are  used  with  various  modifications  in 


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TRANSMISSION  BY  CONTINUOUS  CURRENTS.  81 

nearly  all  continuous  current  dynamos.  Each  has  its  good 
and  bad  features.  The  Gramme  winding  makes  it  very  easy 
to  keep  down  the  voltage  per  segment,  inasmuch  as  for  each 
external  armature  wire  there  is  a  commutator  bar,  while  in 
the  drum  form  there  is  but  one  bar  for  two  wires.  It  is  also 
mechanically  solid  even  when  wound  with  small  wire,  and  no 
two  adjacent  wires  can  have  a  considerable  voltage  between 
them,  thus  making  it  easy  to  build  an  armature  for  high 
E.  M.  F.  On  the  other  hand,  the  drum  winding  gives  a  very 
compact  armature  of  easy  construction,  and  the  magnetism 
induced  in  it  is  less  Hkely  to  disturb  that  of  the  field. 

In  the  small  machines  once  usual,  the  Gramme  type  was  pre- 
ferred for  high  voltages  on  account  of  the  ease  with  which  it 
could  be  repaired,  while  the  drum  was  liked  for  its  simplicity 
of  mechanical  construction  as  a  whole  and  excellent  efficiency 
as  an  inductor.  In  modern  practice  the  differences  between 
these  types  have  become  much  less  marked.  With  large  units, 
particularly  of  the  multipolar  form  now  usual,  the  drum  wind- 
ing is  as  easily  insulated  as  the  Granune,  for  with  the  winding 
now  used  in  such  cases  there  need  be  no  considerable  voltage 
between  adjacent  wire^?,  and  repairs  are  of  very  infrequent 
occurrence.  In  fact,  the  drum  winding  can  be  made  quite  as 
accessible  as  the  other,  and  is  on  the  whole  cheaper  and 
simpler.  Almost  tlie  sole  advantage  of  the  Gramme  (or  ring) 
winding  is  that  of  low  voltage  per  commutator  bar.  Mechan- 
ically, too,  there  is  less  difference  than  formerly,  for  the  coils 
are  in  both  types  generally  bedded  in  slots  in  the  iron  of  the 
armature  core. 

It  must  be  noted  that  the  armature  of  the  modem  dynamo, 
unless  of  small  size  or  unusually  high  voltage,  is  seldom  wound 
with  wire  in  the  ordinary  sense  of  the  word.  Instead,  the 
conductors  are  bars  of  copper,  usually  of  sections  rectangular 
rather  than  round,  and  generally  lacking  any  permanently 
attached  insulation.  Whatever  the  winding,  the  conductors 
on  the  armature  face  are  inclosed  in  close  fitting  tubes  of 
mica  and  specially  treated  paper  or  the  like,  and  then  put 
in  place  on  the  armature  core  or  in  more  or  less  completely 
closed  channelis  cut  in  it.  If  on  the  core  surface,  the  bars  are 
often  not  insulated  on  the  exterior  surface  at  all.     If  the  arma- 


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ELECTRIC   TRANSMISSION  OF  POWER. 


ture  core  be  slotted,  the  insulating  material  is  preferably  put 
in  position  first  and  the  bar  put  in  afterward.  As  to  the  rest 
of  the  winding,  it  is  completed  by  connectors  of  copper  strip  or 
rod  soldered  to  the  face  conductors  and  insulated  in  a  substan- 
tial manner.  Thus  each  convolution,  whether  of  ring  or  drum 
winding,  is  composed  of  from  two  to  four  pieces. 

A  typical  modem  ring  winding  is  shown  in  Fig.  27.  It  well 
exemplifies  the  construction  above  mentioned,  and  in  this  case 
the  uninsulated  faces  of  the  exterior  conductors  form  the  com- 
mutator of  the  machine.     Such  a  construction  of  course  ex- 


FlQ.  27. 


eludes  iron  clad  armatures,  and  is  best  fitted  for  a  machine 
having  a  field  magnet  inside  the  ring  armature.  A  similar 
arrangement  which  avoids  the  above  limitations,  uses  the  side 
connectors  of  the  ring  as  commutator  segments.  The  general 
principle,  however,  is  the  same,  whether  the  commutator 
forms  part  of  the  winding  proper  or  is  a  separate  structure. 

An  iron  clad  drum  winding  of  typical  character  is  shown  in 
Fig.  28.  Here  the  exterior  bars  are  fitted  into  thoroughly 
insulated  slots  in  the  core,  and  wedged  firmly  into  place  by 
insulating  wedges.     Sometimes  the  bars  themselves  are  shaped 


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83 


so  as  to  act  as  wedges.  In  either  case  the  bars  are  held  almost 
as  solidly  as  if  they  formed  an  integral  part  of  the  core.  The 
commutator  in  these  windings  must  be  a  separate  affair.  Fig. 
28  shows  well  the  nature  of  the  winding,  with  its  slotted  core, 
ventilating  spaces,  and  massive  bars  —  in  this  example  4  per 
slot.  The  end  connectors  lie  in  a  pair  of  reverse  spirals,  one 
outside  the  other,  and  separated  by  firm  insulation.  The 
relation  of  these  connectors  to  the  rest  of  the  winding  is  illus- 
trated in  Fig.  25. 

Between  the  modem  drum  and  ring  armatures  it  is  needless 
to  discriminate.  Both  have  been  successfully  used  in  dynamos 
of  the  largest  size,  but  the  iron-clad  drum  is  in  the  more  gen- 
eral use,  while  the  use  of  ring  annatures  is  steadily  declining. 


Fig.  28. 

It  is  very  unusual  to  find  a  standard  generator  of  recent  build 
of  100  KW  or  more  output  with  a  regular  wire  wound  arma- 
ture, and  the  most  of  them  have  some  modification  of  the  bar 
windings  just  described. 

We  have  briefly  reviewed  here  the  armature  windings  at 
present  in  general  use  and  may  now  pass  to  the  various  wind- 
ings employed  for  the  field  magnets.  These  are,  in  continuous 
current  dynamos,  almost  always  connected  with,  and  supplied 
with  current  from,  the  armature  winding,  thus  making  the 
machines  self-exciting.  As  the  armature  is  turned  the  action 
begins  with  the  weak  residual  magnetism  left  in  the  field  mag- 
nets, and  the  current  set  up  by  the  small  E.  M.  F.  thus  produced 
is  passed  around  and  gradually  strengthens  the  magnets,  build- 
ing them  up  to  full  strength.  If  this  residual  magnetism  is 
very  feeble,  as  may  haj)pen  when  it  is  knocked  out  of  the  iron 


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ELECTRIC  TRANSMISSION  OP  POWER. 


by  rough  handling  or  the  continual  jarring  of  a  long  journey, 
it  is  sometimes  quite  difficult  to  get  the  machine  into  action. 

The  simplest  form  of  field  winding,  and  the  one  which  was 
most  extensively  used  at  first,  is  that  in  which  the  current  from 
one  of  the  brushes  passes  around  the  field  magnet  coils  on  its 
way  to  or  from  the  external  circuit  of  the  machine,  as  sho^^^l 
in  Fig.  29.  This  series  winding  possesses  more  than  one  ad- 
vantage. It  consists  of  a  comparatively  small  number  of  eon- . 
volutions  of  rather  large  v^^re  and  so  is  cheap  to  wind,  it  is, 
for  this  same  reason,  little  liable  to  injury  and  easy  to  repair 
when  injured;  and  what  is  of  particular  importance,  whenever 


FlO.  29. 


Kiu.  30. 


the  series  dynamo  is  called  upon  for  more  current,  the  mag- 
netizing power  of  the  field  is  raised  by  the  increase,  thus 
increasing  the  electromotive  force.  This  property,  once  con- 
sidered a  disadvantage,  becomes  of  great  value  in  modem 
windings  adapted  for  the  purpose.  As  the  generation  of 
p].  M.  F.  at  the  start  depends  entirely  on  the  residual  mag- 
netism, series  wound  machines  do  not  *' build  up"  full  voltage 
very  easily  unless  the  resistance  of  the  outside  circuit  is  fairly 
low,  thus  giving  the  current  a  chance. 

The  common  shunt  winding  shown  in  Fig.  30  almost  describes 
itself.  The  brushes  are,  independently  of  the  circuit,  connected 
to  magnetizing  coils  of  relatively  fine  wire.      Although  such  a 


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THANSMLSSION  BY   CONTINUOUS  CURRENTS.  85 

field  wiiuling  i.s  slightly  harder  to  construct  and  to  maintain,  it 
produces  a  magnetic  field  that  is  relatively  free  from  any  actions 
in  the  working  circuit  of  the  machine.  So  long  as  the  E.  M.  F. 
at  the  brushes  is  unaffected  by  changes  of  speed,  the  field  will 
be  quite  steady  except  as  a  very  large  current  in  the  exterior 
circuit  may  reduce  the  voltage  available  for  the  field  by  causing 
a  loss  of  voltage  in  the  armature.  If  the  armature  resistance 
be  very  small,  there  will  be  almost  a  constant  E.  M.  F.  at  the 
brushes  except  as  the  current  flowing  in  the  armature  may 
produce  a  magnetization  opposed  to  the  shunt  field.  For  a 
considerable  time,  then,  the  shunt  winding  was  always  used 
when  a  constant  E.  M.  F.  was  recjuired.  At  the  same  time,  it 
permits  the  E.  M.  F.  to  be  varied,  if  desired,  with  a  very  small 
loss  of  energy,  by  the  simple  expedient  of  putting  a  variable 
resistance  in  circuit  with  the  field  magnets. 

As  the  principles  of  dynamo  construction  became  better 
known,  it  was  apparent  that  the  above  method  of  getting  a 
constant  E.  M.  F.  was  rather  expensive.  To  build  an  armature 
that  would  carry  a  heavy  current  without  noticeable  loss  of 
voltage  and  to  inclose  it  in  fields  so  strong  as  to  be  disturbed 
only  in  a  minute  degree  by  the  magnetizing  effects  of  such 
current,  was  a  task  requiring  much  care  and  a  great  amount  of 
material.  Even  if  this  difficult  problem  were  solved,  the  con- 
stant voltage  would  be  at  the  brushes  of  the  machine  and  not 
at  the  load,  where  it  is  needed. 

An  easy  way  out  of  these  difficulties  is  found  by  considering 
an  important  property  of  the  series-wound  machine  just  men- 
tioned, i.e.y  the  rise  of  E.  M.  F.  as  the  load  on  the  external 
circuit  rises.  If  now  one  takes  a  good  shunt- wound  dynamo 
and  adds  to  the  field  magnets  a  few  series  turns  wound  in  the 
same  direction  as  the  shunt,  the  result  is  an  follows:  At  no 
load,  the  voltage  at  the  brushes  is  that  due  to  the  shunt  alone. 
As  the  load  comes  on  this  voltage  would  naturally  fall  off  by  the 
loss  of  voltage  from  armature  resistance  and  reaction.  The 
series  turns,  however,  at  this  juncture  strengthen  the  field  and 
thus  compensate  for  these  losses.  This  is  the  compound  wind- 
ing^  now  almost  universally  used.  It  is  shown  in  diagram  in 
Fig.  31.  Ordinarily  the  series  tunis  are  more  than  would  be 
needed  for  merely  compensating  the  losses  due  to  armature 


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86 


ELECTRIC  TRANSMISSION  OF  POWER. 


resistance  and  reaction,  so  that  the  voltage  at  the  brushes 
under  load  will  rise  enough  to  make  up  for  the  increased  loss 
in  the  line  due  to  carrying  heavier  current. 

Machines  thus  over-compounded  five  or  ten  per  cent  are  in 
very  common  use. 

The  foregoing  gives  the  rudiments  of  the  machines  used  for 
generating  direct  current.  It  now  remains,  before  taking  up 
the  question  of  power  transmission  proper,  to  consider  briefly 
the  use  of  such  machines  as  motors.  The  underlying  principle 
has  been  already  discussed.     The  power  of  a  motor  to  do 


FlQ.  31. 

work  depends  on  the  stress  of  the  magnetic  field  on  conductors 
carrying  curent  in  it  and  free  to  move.  This  stress  is  virtu- 
ally the  same  as  that  which  has  to  be  overcome  in  using  the 
machine  as  a  generator,  and  reaches  a  very  considerable 
amomit  in  machines  of  any  size. 

In  motors  with  the  field  strengths  often  used,  the  actual 
drag  between  the  field  and  the  armature  wires  may  amoimt  at  a 
rough  approximation,  to  nearly  an  ounce  pull  on  each  foot  of 
conductor  in  the  field  for  every  ampere  flowing  through  the 
wire.  With  a  20  HP  motor  the  actual  twisting  effort  or  torque 
at  the  surface  of  the  armature  might  easily  be  considerably 
over  a  hundred  pounds  pull.     Forces  of  this  size  emphasize 


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TRANSMISSION  BY  CONTINUOUS  CURRENTS.  87 

the  need  of  solid  armature  construction,  with  the  conduc- 
tors firmly  locked  in  place,  particularly  since  the  magnetic 
drag  is  not  steady,  but  conies  somewhat  violently  u[)on  the 
conductors  as  they  enter  the  field.  With  the  old  smooth  core 
armatures  wound  with  wire,  the  conthictors  not  infretiuently 
worked  loose  and  chafed  each  other,  and  even  the  entire  wind- 
ing has  been  known  to  slip  on  the  core.  In  modern  windings, 
either  iron-clad  or  modified  smooth  core,  such  accidents  are 
nearly  impossible. 

When  the  armature  conductors  of  the  motor  cut  through  its 
field  as  the  armature  revolves,  an  electromotive  force  is  neces- 
sarily generated  in  them  as  in  every  other  case  when  the 
magnetic  forces  on  a  conductor  change.  There  is  thus  pro- 
duced, as  a  necessary  part  of  the  action  of  every  motor,  a 
counter  electromotive  force  in  the  armature.  This  electro- 
motive force  plays  a  very  important  part  in  the  internal 
economy  of  the  motor. 

In  the  first  place,  the  magnitude  of  the  counter  electromo- 
tive force  determines  the  amount  of  current  that  can  flow 
through  the  motor  when  supplied  at  a  given  voltage.  The 
resistance  of  the  armature  from  brush  to  brush  may  be  only 
a  few  thousandths  or  even  ten  thousandths  of  an  ohm, 
while  the  applied  voltage  may  be  several  hundred  volts.  The 
resulting  current,  however,  is  not  that  which  would  flow 
through  the  given  resistance  under  the  pressure  applied,  but 
the  flow  is  determined  by  the  difference  between  the  applied 
electromotive  force  and  the  counter  E.  M.  F.  of  the  motor,  so 
that  in  starting  a  motor  when  the  armature  is  at  rest  and  there 
is  therefore  no  counter  E.  M.  F.,  a  resistance  must  be  inserted 
outside  the  armature  to  cut  do^vn  the  initial  riLsh  of  current. 

In  the  second  place,  the  counter  electromotive  force  mea- 
sures the  output  of  the  motor  for  any  given  current.  It  does 
this  because  the  very  same  things,  i.e.,  strength  of  field, 
amount  of  wire  under  induction,  and  speed,  which  determine 
the  output  for  a  given  current,  also  determine  the  magnitude 
of  the  counter  electromotive  force. 

Therefore,  when  the  machine  is  running  as  a  motor,  while 
the  energy  supplied  to  it  is  the  product  of  the  voltage  by  the 
amperes  which  flow  through  the  armature,  the  output  of  the 


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88  ELECTRIC  TRANSMISSION  OF  POWER. 

motor  is  determined  by  the  product  of  the  counter  electromo- 
tive force  into  the  selfsame  current;  hence,  under  given  con- 
ditions, the  ratio  between  the  impressed  and  counter  electro- 
motive forces  of  the  motor  determines  the  efficiency  of  the 
motor.  The  difTerencc  between  these  electromotive  forces 
determines  the  input  of  energy,  since  it  determines  the  cur- 
rent which  may  flow;  therefore,  as  the  counter  electromotive 
force  increases,  the  efficiency  of  the  motor  increases,  but  the 
output  is  limit<jd  by  the  decreased  input. 

With  a  fixed  electromotive  force  supplied  to  the  armature, 
the  output  of  the  motor  per  ampere  of  current  will  diminish  as 
the  counter  electromotive  force  diminishes,  but  the  total 
amperes  flowing  will  increase  because  the  difference  between 
the  applied  and  counter  E.  M.  F.  has  also  increased.  Thus 
the  total  output  increases,  although  at  a  lower  efficiency, 
when  the  counter  E.  M.  F.  decreases.  Since  the  input  (which 
is  determined  by  the  difTerencc  between  counter  and  applied 
E  .M.  F.'s)  nmltiplied  by  the  efficiency  (which  is  determined 
by  the  counter  E.  M.  F.)  equals  the  net  output  of  the  motor, 
this  output  will  be  at  a  maximum  when  the  counter  E.  M.  F. 
and  the  effective  E.  M.  F.  are  equal  to  each  other.  This  fol- 
lows from  the  general  law,  that  the  product  of  two  quantities, 
the  sum  of  which  is  fixed,  will  be  a  maximum  when  these 
quantities  are  c<iual. 

It  must  be  distinctly  understood,  however,  that  at  this  point 
of  theoretical  maxinunn  output  the  motor  is  very  inefficient, 
and  that  mechanical  considerations  prevent  the  efficiency 
being  wholly  determined  by  the  counter  E.  M.  F.,  while  spark- 
ing and  heating  generally  j)revcnt  working  with  the  counter 
E.  M.  F.  equal  to  the  effective  E.  M.  F. 

In  actual  practice  motors  are  worked  under  very  diverse 
conditions,  and  some  of  these  it  is  worth  while  to  take  up  in 
detail,  following  the  preceding  generalizations.  The  energy 
may  be  supplied  at  constant  current,  at  constant  voltage,  with 
neither  current  nor  voltage  constant,  at  fixed  or  variable 
speed,  and  subject  to  a  wide  variety  of  conditions;  the  motors 
may  be  wound  either  series,  shunt,  compound,  or  with  various 
modifications  of  these  windings,  and  may  be  either  self  regu- 
lating with  respect  to  various  requirements,  or  regulated  by 


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TRANSMISSION  BY  CONriNUOUS  CURRENTS.  89 

extraneous  means.  In  the  ordinary  i)roblenis  dealt  with  in 
power  transmission,  these  conditions  may  be  classified  in  a 
fairly  simple  way  as  follows  : 

Case  I.   Series-wound  motors  at  constant  current. 

Case  II.    Series-wound  motors  at  constant  voltage. 

Case  III.  Series-wound  motors  with  interdependent  current 
and  voltage. 

Case  IV.   Shunt-wound  motore  at  constant  voltage. 

The  first  class  is  now  rarely  found  in  practice,  and  is  of 
real  commercial  importance  only  in  a  few  cases.  The  second 
class  is  very  widely  used  in  a  particular  case,  to  wit:  electric 
railway  service,  and  consequently  it  is  of  great  practical  im- 
portance. The  third  class  of  motors  is  used  occasionally  with 
great  success  but  not  very  extensively,  while  the  fourth  includes 
the  vast  majority  of  all  the  continuous  current  machines 
running  for  purposes  other  than  electric  railway  service. 
These  latter  cases,  therefore,  it  is  worth  while  to  take  up 
somewhat  thoroughly. 

Case  I.  —  Series-wound  motors  operated  with  a  constant  cur- 
rent originally  came  into  use  in  connection  with  arc  lighting 
circuits,  which  for  some  years  formed  the  most  generally 
available  source  of  current.  Such  lines  are  fed  from  dynamos 
in  which  the  current  is  kept  constant  by  special  regulation, 
while  the  voltage  rises  and  falls  in  accordance  with  the  load, 
consisting  of  lamps  or  motors  in  scries  with  each  other.  We 
are  therefore  relieved  of  any  concern  about  the  current,  since 
it  is  kept  constant  (juite  irrespective  of  what  happens  in  the 
motor. 

Under  these  circumstances,  in  a  series-wound  motor,  the 
torque  will  be  constant,  since  the  field  is  constant,  and  the 
output  of  the  motor  will  vaiy  directly  with  the  speed.  If  it 
be  loaded  beyond  its  capacity,  it  simply  refuses  to  start  the 
load,  inasmuch  as  its  tor<|Uc  is  limited  by  the  current.  If  it 
starts  w^ith  a  load  within  its  limit  of  torque,  its  speed  will 
steadily  increase  until  that  limit  is  reached.  This  may  be 
comparatively  soon  if  the  load  is  a  rapidly  increasing  one,  or 
the  machine  may  race  until  its  own  friction  of  air  and  bearings, 
magnetic  resistances,  and  the  induction  of  idle  currents  in  the 
core  and  frame  serve  to  furnish  resistance  up  to  its  limit  of 


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90  ELECTRIC  TRANSMISSION  OF  POWER. 

torque.  When  running  at  a  given  speed,  any  increase  of  load 
causes  the  speed  to  fall  off,  while  decrease  of  load  produces 
racing.  Unless  these  tendencies  are  controlled,  this  type  of 
machine  becomes  almost  useless  for  practical  purposes,  as 
regularity  of  speed  under  change  of  load  is  generally  highly 
desirable.  In  fact,  the  tendency  to  run  at  constant  torque  is 
generally  inconvenient.  To  obviate  this  very  serioius  difficulty, 
various  devices  have  been  tried  with  tolerable  success.  The 
commonest  is  to  vary  the  torque  in  accordance  with  the  load 
by  changing  the  field  strength,  or  by  shifting  the  brushes  so  as 


FlO.  32. 

to  throw  the  armature  coils  out  of  their  normal  relation  to  the 
magnetic  field. 

Since  the  object  of  such  changes  is  to  vary  the  output  at 
constant  current,  and  since  this  output  is  measured  by  the 
counter  E.  M.  F.  of  the  motor,  the  real  problem  of  such  regu- 
lation is  to  vary  the  counter  E.  M.  F.  in  proportion  to  the 
output  desired.  Therefore,  the  same  general  means  that 
serve  to  accomplish  this  end  in  an  arc  dynamo,  keeping  the 
current  constant  and  varying  the  E.  M.  F.,  will  serve  to  regu- 
late the  corresponding  motor. 

As  in  this  case  the  speed  is  the  thing  to  be  held  constant, 
the  usual  means  taken  for  working  the  regulating  devices  is  a 


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TRANSMISSION  BY  CONTINUOUS  CURRENTS.  91 

centrifugal  governor,  which  generally  acts  to  shift  the  brushes 
or  to  put  in  circuit  more  or  less  of  the  field  winding,  which  for 
this  purpose  is  divided  into  sections.  In  still  other  arrange- 
ments the  governor  acts  to  slide  the  armature  partially  into  or 
out  of  the  field,  or  to  work  a  rheostat  which  shunts  the  field 
magnet,  as  in  the  Brush  regulator  for  constant  current.  An 
excellent  example  of  a  small  constant  current  motor  regulated 
on  the  last  mentioned  principle  is  shown  in  Fig.  32. 

As  to  the  operation  of  these  regulating  devices,  it  is  tolerably 
good  if  everything  is  carefully  looked  after  and  kept  in  adjust- 
ment. The  efficiency  of  such  motors  is  not  generally  as  high 
as  that  of  other  types  at  light  loads,  owing  to  the  nearly  con- 
stant loss  in  the  armature  due  to  constant  current  working. 
At  and  near  full  load  the  efficiency  may  be  good. 

In  addition,  the  current  is  highly  dangerous,  coming  as  it 
does  from  generators  of  very  high  voltage,  and  even  the  voltage 
across  the  brushes  is,  in  machines  of  any  size,  sufficient  to 
give  a  dangerous  or  even  fatal  shock.  A  10  HP  motor,  for 
example,  on  the  customary  10-ampere  circuit,  would  have  a 
difference  of  potential  of  about  800  volts  between  the  brushes 
at  full  load.  As  a  few  such  motors  would  load  even  the  largest 
arc  dynamos,  besides  being  dangerous  in  themselves,  opera- 
tions have  generally  been  confined  to  smaller  units.  On 
account  of  the  danger  and  the  mechanical  and  other  difficulties, 
the  arc  motor  has  come  to  be  looked  upon  as  a  last  resort,  is 
seldom  or  never  used  when  anything  else  is  available,  and,  to 
the  credit  of  the  various  manufacturers  be  it  said,  is  nearly 
always  sold  and  installed  with  a  specific  explanation  of  its 
general  character  and  the  precautions  that  must  be  taken 
with  it. 

In  spite  of  all  these  objections,  the  constant  current  motor 
often  does  good  and  steady  work,  and  some  such  motors  have 
been  used  for  years  without  accident  or  serious  trouble  of  any 
kind.  They  have  been  employed,  however,  only  sparingly  for 
power  transmission  work  of  any  kind,  and  when  so  used  are 
mostly  on  special  circuits  of  50  to  150  amperes. 

Case  II.  —  Series  motors  worked  at  constant  potential  are 
very  widely  used  for  electric  railway  service  and  other  cases, 
such  as  hoisting,  in  which  great  variations  of  both  speed  and 


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92  ELPJCTRIC  TRANSMISSION  OF  POWER. 

torque  are  desirable.  When  supplied  at  constant  potential,  the 
speed  of  a  series-wound  motor  varies  widely  with  the  load.  In 
any  case  the  speed  increases  until  the  counter  E.  M.  F.  rises 
high  enough  to  cut  the  current  down  to  the  amount  necessary 
to  give  the  torque  sufficient  for  that  load  and  speed. 

If  the  field  be  strengthened,  the  motor  will  give  a  certain 
output  at  a  lower  speed  than  before;  if  it  be  weakened,  at  a 
higher  speed;  the  torque  behig  in  these  cases  correspondingly 
increased  or  decreased. 

The  tonjue  increases  rapidly  with  the  current,  so  that  when 
the  counter  E.  M.  F.  is  small,  or  zero,  as  in  starting  from  rest, 
the  torque  is  very  great,  a  property  of  immense  value  in  start- 
ing heavy  loads.  For  m  starting,  not  only  is  the  current 
through  the  armature  large,  but  the  field  is  at  its  maxinmm 
strength.  If  the  field  strength  varied  directly  as  the  current, 
the  torque  would  vary  nearly  as  the  square  of  the  current. 

As  a  rule,  however,  these,  like  most  other  motors,  are  worked 
with  a  fairly  intense  magnetization  of  the  fields,  so  that  doub- 
luig  the  magnetizing  current  b}-  no  means  doubles  the  strength 
of  the  field.  In  fact,  most  series  motors  for  constant  potential 
circuits  are  of  the  type  used  for  electric  railways,  and  wound 
so  that  the  field  magnets  are  nearly  saturated  even  with  very 
moderate  currents.  Hence  the  torque  in  such  cases  increases 
but  a  trifle  faster  than  the  current.  This  construction  is 
adopted  in  order  to  reduce  the  amount  of  iron  necessary  to 
secure  a  given  strength  of  field,  and  so  to  lighten  and  cheapen 
the  motor. 

It  is  quite  obvious  that  while  series  motors  at  constant 
potential  have  the  advantage  of  being  able  to  give  on  occasion 
very  great  tonjue,  they  suffer  from  the  same  disadvantage  as 
constant  current  motors,  in  that  they  are  not  self-regulatuig 
for  constant  speed.  Centrifugal  governors  could,  of  course, 
be  applied  to  them,  but  since  it  happens  that  most  work 
requiring  great  torque  also  requires  variable  speed,  nothing 
of  the  kind  is  usually  necessary. 

As  previously  explained  the  speed  can  be  easily  regulated  to 
a  certain  extent  by  changing  the  field  strength,  thus  changing 
the  counter  E.  M.  F.,  but  owing  to  the  peculiarity  of  design 
just    noted,    this    method   is   rather   ineffective,  requiring   a 


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TRANS^fISSION  BY  CONTINUOUS  CURRENTS. 


93 


great  change  in  the  field  winding  for  a  moderate  change  in 
speed. 

In  general,  when  a  considerable  range  of  speed  is  needed, 
constant  potential  working  is  abandoned  and  the  speed  is 
changed  by  varying  the  impressed  E.  M.  F.  by  means  of  a 
rheostat.  If  this  E.  M.  F.  be  lowered,  the  current  decreases 
and  the  speed  sags  off  imtil  the  new  counter  E.  M.  F.  is  low 
enough  to  let  pass  just  enough  current  to  maintain  the  output 
at  the  reduced  speed.  When  the  applied  E.  I\I.  F.  is  increased 
the  reverse  action  takes  place.     Under  these  circumstances,  for 


Km.  33. 


a  fixed  load  the  current  is  approximately  the  same,  independent 
of  the  speed;  for  with  a  uniform  load  the  torque  is  constant, 
while  the  output  (i.e.,  rate  of  driving  the  load)  varies.  Many 
railway  motors  are  regulated  in  the  manner  just  described, 
although  in  addition  the  field  strength  is  sometimes  varied  by 
cutting  out  or  recombining  fields  and  by  series  parallel  control. 
Rheostatic  control  necessarily  wastes  energy,  and  the  greatest 
recent  improvement  in  railway  practice  consists  in  reducing 
the  E.  M.  F.  applied  to  the  car  motors  by  throwing  them  in 
series.  This  secures  a  low  speed  economically,  though  the 
rheostat  still  comes  into  play  at  intermediate  speeds. 


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94  ELECTRIC  TRANSMISSION  OF  POWER, 

Speaking  broadly  then,  series-wound  motors,  while  possess- 
ing many  valuable  properties,  are  limited  in  their  usefulness 
by  their  tendency  to  vary  'widely  in  speed  when  the  load 
changes.  Hence  they  are  used  chiefly  in  cases  where  the 
speed  is  to  be  varied  deliberatel3^  A  typical  early  motor  of 
this  class,  used  for  hoists  and  the  like,  with  rheostatic  control, 
is  shown  in  Fig.  33. 

In  spite  of  the  difficulty  in  regulation,  the  series  motor  pos- 
sesses some  considerable  advantages:  The  field  coils  being  of 
coarse  wire  are  easily  and  cheaply  wound,  even  in  motors  for 
very  high  voltage;  the  same  quick  response  to  changes  in 
current  or  load  that  makes  it  hard  to  obtain  uniform  speed  is 
also  most  important  in  many  kinds  of  work;  the  powerful 
initial  torque  Is  coupled  with  the  useful  property  of  prompt 
reversal.  All  these  make  the  series  motor  preiMiunent  for  cer- 
tain purposes,  especially  where  severe  work  is  to  be  coupled 
with  hard  usage. 

There  is  one  case,  too,  in  which  the  series-wound  motor  can 
be  made  accurately  self-regulating  for  constant  speed  —  a  case 
somewhat  peculiar  and  unusual,  but  yet  worthy  of  special 
attention. 

Case  III. — We  have  seen  that  when  the  load  on  a  series 
motor  supplied  at  a  certain  voltage  increases,  the  speed  falls 
ofT  until  the  increasing  current  due  to  the  lessened  counter 
E.  M.  F.  raises  the  torque  sufficiently  to  meet  the  new  con- 
ditions. Imagine  now  the  impressed  P].  M.  F.  to  be  so  varied 
that  the  slightest  increase  of  current  in  the  motor  is  met  by 
a  rise  in  the  E.  M.  F.  applied  to  it.  Evidently  the  speed 
would  not  have  to  fall  as  before,  for  the  greater  applied  voltage 
would  furnish  ample  current  for  all  the  needs  of  the  load.  If 
the  variation  in  voltage  could  be  made  to  depend  on  change 
of  torque,  not  giving  the  speed  time  to  change,  the  regulation 
would  be  almost  perfect.  Such  a  method  has  been  proposed, 
but  owing  to  mechanical  difficulties  has  not  been  used  to  any 
extent. 

It  is  possible,  however,  so  to  combine  a  special  motor  and 
generator  that  the  former  will  be  very  closely  uniform  in  speed, 
quite  independent  of  the  load.  In  this  connection  we  must 
revert  to  the  properties  of  the , series- wound  dynamo.     If  such 


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95 


a  machine  be  driven  at  constant  speed  its  electromotive  force 
will  increase  with  the  current,  since  the  strength  of  field, 
here  the  only  variable  factor  in  the  voltage,  will  increase  with 
the  current.  If  the  field  magnets  of  the  generator  are 
unsaturated,  that  is,  not  so  strongly  magnetized  as  to  require 
considerable  current  to  produce  a  moderate  increase  of  mag- 
netization, they  will  respond  very  promptly  to  an  increase  of 
load  by  raising  the  voltage.  If  such  a  generator  be  connected 
to  a  series- wound  motor  of  proper  design,  the  pair  will  work 
together  almost  as  if  connected  by  a  belt  instead  of  a  long 
line,  and  the  motor  will  nm  at  a  nearly  uniform  speed,  since 


Fia.  34. 

the  least  diminution  of  speed,  with  its  accompanying  increase 
of  current,  will  be  met  by  a  rise  in  the  voltage  of  the  genera- 
tor.    Such  an  arrangement  is  shown  in  diagram  in  Fig.  34. 

In  this  cut  A  is  the  generator  supplying  current  to  the 
motor  B.  The  machines  should  be  of  practically  the  same 
capacity,  for  the  generator  cannot  supply  current  except  to  the 
one  motor  without  disturbing  the  regulation.  Whenever  the 
load  on  B  changes,  a  very  small  reduction  in  speed  suffices  to 
raise  the  voltage  of  A  and  thereby  to  hold  up  the  speed  of  B. 
To  this  end  the  field  magnets  of  B  must  be  more  strongly 
saturated  than  those  of  A,  else  the  same  increase  of  current 
will  raise  the  counter  E.  M.  F.  of  the  motor  and  defeat  the 
purp<»se  of  the  c<)m])ination.     If  the  fielcis  of  the  two  machines 


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9&  ELECTRIC  TRANSMISSION  OF  POWER. 

are  properly  designed,  the  generator  will  increase  its  voltage 
under  increasing  load  just  enough  to  hold  the  motor  at  speed, 
as  a  very  slight  change  in  current  immediately  reacts  on  the 
generator. 

It  is  even  possible  to  make  the  motor  rise  in  speed  under 
load  if  the  generator  is  sufficiently  sensitive  to  changes  of  cur- 
rent. This  is  generally  needless,  but  it  is  often  useful  so  to 
design  A  and  B  that  the  former  will  rise  in  voltage  fast  enough 
not  only  to  compensate  for  the  added  load  on  the  motor  but 
for  the  added  loss  of  energy  in  the  line,  entailed  by  the  in- 
crease of  current,  thus  regulating  the  motor  even  at  a  long 
distance.  The  difference  of  saturation  between  the  generator 
and  motor  fields  new!  not  involve  material  difference  of  design, 
since  it  may  be  effected  by  shunting  the  motor  field.  When 
properly  adjusted,  the  system  is  capable  of  holding  the  motor 
speed  constant  within  two  per  cent  through  the  range  of  load 
for  which  the  machines  are  planned. 

It  should  be  noted  in  connection  with  Fig.  34  that,  whereas 
the  current  circulating  in  the  armature  of  a  generator  tends  to 
disturb  the  magnetic  field  in  one  direction,  in  a  motor  the  same 
reaction  is  in  the  opposite  direction.  For  the  current  in  the 
motor  is  driven  through  the  armature  against  the  coimter 
E.  M.  F.,  i.e.,  in  the  direction  opposite  to  that  of  the  current 
the  machine  would  give  if  running  as  a  generator.  As  the 
effect  of  the  reaction  is  to  skew  the  direction  of  the  magnetic 
field  that  affects  the  armature  conductors,  and  the  conunuta- 
tion  must  take  place  when  the  commuted  coil  is  not  imder  a 
varying  induction,  the  armature  reaction  compels  one  to  shift 
the  brushes  slightly  away  from  the  position  they  would  have  if 
the  field  were  perfectly  symmetrical.  This  shifting  is  in  the 
direction  of  armature  rotation  in  a  generator,  but  for  the 
reason  above  noted  has  the  opposite  direction  in  a  motor.  In 
either  case  it  need  be  only  a  few  degrees. 

Case  IV.  —  Shunt-wound  motors'  are  almost  invariably 
worked  on  constant  potential  circuits,  to  which  they  are  partic- 
ularly well  suited.  They  form  by  far  the  largest  class  of  motors 
in  general  use  and  owe  this  advantage  mainly  to  their  beautiful 
self-regulating  properties. 

The  shunt  motor  is  in  construction  practically  the  same  as 


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TRANSMISSIOX  BY  CONTiyUOUS  CURREXTS.  97 

a  shunt-wound  dynamo.  Let  us  look  into  the  action  of  such  a 
machine  when  supplied  from  a  source  of  constant  voltage.  If 
the  design  be  reasonably  efficient,  the  armature  will  have  a 
very  low  resistance  and  the  shunt  circuit,  which  includes  the 
field  coils,  a  resistance  several  himdred  times  greater.  When 
such  a  machine  is  supplied  with  current  of  constant  voltage  at 
its  brushes  and  is  running  at  any  given  speed  and  load,  the 
current  through  the  armature  is  practically  determined  by  the 
counter  E.  M.  F.  developed,  the  armature  resistance  being 
almost  negligible.  The  shunt  is  of  high  resistance  and  takes 
a  certain  small  amount  of  current,  determined  by  the  voltage 
across  the  brushes.  Now  let  the  load  increase;  the  field  is, 
aside  from  loss  of  voltage  on  the  line,  practically  constant,  and 
the  first  effect  of  the  added  load  is,  as  in  a  series  motor,  to 
reduce  the  speed.  But  this  lowers  the  coimter  E.  M.  F.,  and 
consequently  the  armature  current  rises  and  the  torque  is 
increased,  thereby  enabling  the  motor  to  operate  under  the 
larger  load.  The  torque  necessary  to  enable  the  motor  to 
maintain  an  increased  load  varies  directly  as  the  load  and  is 
also  directly  proportional  to  the  current.  But  since  the  cur- 
rent is  closely  proportional  to  the  difference  between  the 
impressed  and  counter  E.  M.  F.'s,  it  is  possible  to  design  a 
machine  so  as  to  run  at  almost  exactly  constant  speed. 

The  constancy  depends  really  on  the  armature  resistance, 
small  as  it  is.  For  example,  a  motor  is  designed  to  run  at  100 
volts.  Running  light  the  comiter  E.  M.  F.  is  99.9  volts,  and 
with  an  armature  resistance  of  0.01  ohm  the  current  will  be  10 
amperes.  The  work  done  is  say  1  HP.  Now  let  a  full  load, 
say  20  HP,  be  thrown  on.  The  torque  will  have  to  be  in- 
creased 20  times,  requiring  200  amperes.  But  this  will  flow 
through  the  armature  under  an  effective  pressure  of  2  volts. 
Hence  the  counter  E.  M.  F.  will  only  have  to  fall  to  98  volts  to 
provide  current  enough  to  meet  the  new  condition.  As  the 
coxmter  E.  M.  F.  varies  directly  as  the  speed,  a  fall  in  speed  of 
less  than  2  per  cent  will  follow  the  increase  of  load.  This 
computation  neglects  all  questions  of  armature  reaction  as 
well  as  the  effect  of  this  minute  fall  in  speed  on  the  output, 
but  fairly  represents  a  case  that  might  actually  be  met  with  in 
the  best  modem  practice.    In  fact,  shunt  motors  have  been  so 


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98  ELECTRIC  TRANSMISSION  OF  POWER, 

designed  as  to  vary  no  more  than  li  per  cent  in  speed  from 
no  load  to  full  load.  A  variation  of  5  or  6  per  cent  is,  how- 
ever, more  usual. 

When  supplied  from  an  over  compounded  generator  so  that 
the  impressed  voltage  may  increase  with  the  load,  a  shunt 
motor  can  be  operated  even  more  closely  to  constant  speed 
than  indicated  above,  since  there  is  no  longer  need  for  a  fall 
in  speed  to  maintain  the  requisite  difference  between  the 
impressed  and  counter  E.  M.  F.'s.  In  such  case  any  tendency  to 
fall  in  speed  is  at  once  corrected  by  the  rise  in  voltage  on  the 
line.  This  scheme  is  seldom  used,  however,  since  it  is  ill  fitted 
for  simultaneously  operating  a  number  of  motors  at  varying 
loads,  and  for  single  units  has  no  particular  advantages  over 
the  series-wound  pair  previously  noted,  or  a  very  simple 
arrangement  of  alternating  apparatus. 

Not  only  can  the  shunt  motor  be  worked  at  nearly  constant 
speed,  but  it  also  has  the  advantage  of  permitting  a  consid- 
erable range  of  sp)eed  variation  without  sacrificing  much  in  the 
matter  of  efficiency.  We  have  already  seen  that  a  change  in 
field  strength  involves  a  change  of  speed,  since  it  necessarily 
alters  the  counter  E.  M.  F.,  which  in  turn  modifies  the  current. 

In  a  shunt  motor  the  immediate  effect  of  a  decrease  of  field 
strength  is  to  lower  the  counter  E.  M.  F.,  letting  more  current 
through  the  armature  and  increasing  the  torque.  Hence,  the 
speed  rises  until  the  current  and  torque  adjust  themselves  to 
the  requirements  of  the  load.  On  the  other  hand,  if  the  field 
be  strengthened,  the  current  necessary  to  carry  the  load  can- 
not be  obtained  without  a  fall  in  speed.  It  is  clear  that  the 
changes  of  speed  thus  obtained  may  be  quite  considerable,  for 
in  a  motor  such  as  that  just  described  a  variation  of  10  per 
cent  in  the  field  would  produce  an  immense  variation  of  cur- 
rent, which  would  have  to  be  compensated  by  a  change  in 
speed  as  great  as  the  change  in  the  field.  Inasmuch  as  these 
field  changes  can  be  produced  by  varying  the  field  current, 
which  is  always  small,  through  a  rheostat  in  the  circuit,  this 
change  of  field  strength  can  be  accomplished  with  but  a  tri- 
fling waste  of  energy.  If  the  field  magnets  are  comparatively 
unsaturated,  it  is  not  difficult  to  obtain  perhaps  50  per  cent 
variation  in  speed.    A  motor  designed  for  such  work  is,  how- 


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99 


ever,  bulky,  as  it  must  if  necessary  be  possible  to  get  torque 
enough  to  handle  the  full  load  with  a  field  much  below  its 
normal  strength. 

It  should  be  noted  that  even  when  running  at  a  considerably 
modified  speed,  the  motor  must  still  be  nearly  self-regulating 
for  changes  in  load,  for  the  conditions  that  govern  self-regu- 
lation are  within  moderate  limits  unaffected  by  the  particular 
strength  of  field  employed.  Only  when  the  armature  reaction 
has  been  greatly  modified  will  the  regulation  be  sensibly 
disturbed. 


Fig.  35. 

A  device  sometimes  used  to  improve  the  regulation  of  motors 
essentially  shunt  wound  is  the  so-called  differential  winding. 
This  consists  of  an  additional  field  winding  in  series  with  the 
armature,  but  around  which  the  current  flows  in  such  a  direc- 
tion as  to  demagnetize  the  field.  The  total  field  strength  is 
then  due  to  the  difference  between  the  magnetizing  power  of 
the  shunt  and  of  this  regulating  coil.  When  the  load  on  the 
motor  increases,  the  additional  current  due  to  a  minute  change 
of  speed  will  weaken  the  field,  and  thence  cause  the  motor  to 
run  faster  until  the  counter  E.  M.  F.  adjusts  the  current  to'the 
new  speed  and  output.  Differential  winding  obviously  requires 
an  extra  expenditure  of  energy  in  the  field,  since  the  shunt  and 


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100 


ELECTRIC  TRANSMISSION  OP  POWER. 


series  turns  act  against  each  other.  Fig.  35  shows  the  Sprague 
motor  wound  on  this  differential  plan,  now  only  of  historical 
interest,  but  which  through  its  good  qualities  did  much  to 
popularize  the  electric  motor  in  America.  Plate  II  shows 
in  Fig.  1  a  Westinghouse  bi-polar  shunt  motor,  and  in  Fig. 
2  a  G.  E.  six-pole  shunt  motor  for  slow  speed. 

Various  modifications  of  shunt-  and  series-wound  motors 
have  from  time  to  time  appeared,  devised  for  particular  adapta- 
tion to  special  purposes  or  sometimes  merely  for  the  sake  of 
novelty.  None  of  them,  however,  are  of  sufficiently  general 
importance  to  find  a  place  here  except  a  single  ver}'^  beautiful 
method  of  obtaining  efficiently  a  very  wide  range  of  speed. 

The  principle  of  this  method  is  to  work  the  motor  at  normal 


Fig.  36. 

full  excitation,  but  to  deliver  to  the  armature  a  current  of 
variable  E.  M.  F.  so  that  a  given  current  and  hence  torque 
may  accompany  very  various  values  of  the  counter  E.  M.  F. 
Fig.  36  shows  the  connections  employed  to  effect  this  result. 
Here  C  is  the  working  motor,  B  the  special  generator  which 
feeds  its  armature,  A  the  motor  used  to  drive  this  generator, 
and  D  the  rheostat  and  reversing  switch  in  the  generator  field 
circuit  which  allows  the  generator  E.  M.  F.  to  be  varied.  In 
the  figure  the  motor  //  is  shown  as  a  synchronous  alternating 
machine  with  a  commutator  from  which  are  fed  the  fields  of 
the  three  machines.  In  ordinary  central  station  practice  A  is 
a  continuous  current  motor,  and  the  fields  are  fed  direct  from 
the  distributing  mains.  The  result  of  this  arrangement  is  that 
the  motor  field  C  remains  at  full  strength,  while  the  armature 


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Fio.  1. 


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1M.ATE   II. 


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TRANSMISSION  BY  CONTINUOUS  CURRENTS.        101 

current  can  be  brought  to  any  required  strength  at  any  desired 
armature  speed  within  a  very  wide  range.  Hence,  C  can  give 
full  load  torque  or  even  more  while  the  armature  is  merely 
turning  over  a  few  times  per  minute,  and  the  speed  can  be 
brought  up  with  the  utmost  delicacy  and  held  at  any  desired 
point.  And  at  every  speed  the  motor  holds  its  speed  fairly 
well,  irrespective  of  changes  of  load.  For  elevators,  hoists, 
and  similar  work  this  device  is  extremely  useful.  The  only 
objection  to  it  is  the  cost  of  installing  the  two  extra  machines, 
which  is  of  course  considerable.  Nevertheless  the  regulation 
attained  is  so  beautiful  and  perfect  that  the  cost  often  becomes 
a  minor  consideration,  and  the  device  is  very  widely  used  in 
cases  where  variable  speed  is  essential. 

POWER    TRANSMISSION    AT   CONSTANT    CURRENT. 

In  its  general  aspect  this  method  must  now  be  regarded  as  a 
makeshift.  It  came  into  existence  at  a  time  when  the  only 
circuits  extensively  installed  were  those  for  arc  lighting,  and 
hence,  if  motors  were  to  be  used  at  all,  they  must  needs  be  of 
the  constant  current  type.  As  incandescent  lighting  became 
more  common  the  arc  motors  were  gradually  replaced  by  shunt 
motors  worked  at  constant  potential.  A  few  constant  current 
plants  especially  for  motor  service  have  been  installed  both 
here  and  abroad,  but  for  the  most  part  they  have  merely 
dragged  out  a  precarious  existence,  and  in  this  coimtry  have 
been  abandoned. 

There  is  good  reason  for  this.  The  motors  usually  regulate 
indifferently,  and  there  is  serious  objection  to  running  high 
voltage  wires  into  buildings  when  it  can  be  avoided. 

The  objections  of  the  insurance  companies  alone  are  quite 
sufficient  to  discourage  the  practice.  The  constant  current 
has  often  been  advocated  for  long  distance  transmission  of 
power  where  high  voltage  is  a  necessity.  For  such  service  the 
method  has  the  great  advantage  that  the  motors  do  not  need 
extraordinary  insulation  except  from  the  ground.  A  constant 
potential  service  at  5,000  volts  continuous  current  would  be 
utterly  impracticable,  if  distribution  of  power  in  moderate 
units  were  to  be  attempted,  while  with  constant  currents  it 


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102  ELECTRIC  TRANSMISSION  OF  POWER, 

is  entirely  feasible,  although  objectionable  on  the  grounds 
mentioned.  In  addition,  unless  a  proposed  transmission  be 
for  power  alone,  the  constant  current  method  shares  with  con- 
stant potential  of  high  voltage  the  very  grave  difficulty  that 
an  incandescent  lamp  service  is  out  of  the  question,  without 
secondary  transformation  of  the  necessarily  high  line  voltage 
to  a  very  moderate  pressure.  This  is  somewhat  expensive  with 
continuous  currents  of  any  kind,  and  at  once  introduces  the 


Fio.  :J7. 

troublesome  question  of  regulation  at  constant  current  into 
the  problem. 

To  reduce  the  energy  sent  over  a  high  voltage  continuous 
current  Hne  to  a  pressure  at  which  incandescent  lamps  can  l)e 
fed,  two  methods  are  possible.  We  may  pass  by  the  plan 
of  using  many  lamps  in  series  as  of  very  Hmited  appli- 
cability and  forbidden  by  the  fire  underwriters.  First,  the 
required  power  may  be  received  by  a  motor  of  appropriate 
size,  which  is  belted  or  coupled  to  a  low  voltage  generator. 
This  device  does  the  work,  but  it  involves  installing  three 
times  the  capacity  of  the  lamps  desired  in  machinery  of  a 
somewhat  costly  character,  and  losing  in  the  motor  and  gen- 


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TRANSiMISSION  BY  CONTINUOUS  CURRENTS.        103 

erator  perhaps  15  or  20  per  cent  of  the  energy  supplied  from 
the  line.  The  other  alternative  is  to  employ  a  composite 
machine  combining  the  functions  of  motor  and  generator. 
This  piece  of  apparatus  is  variously  known  as  a  motor-genera- 
tor, dynamotor,  or  continuous  current  converter.  It  is  a 
dynamo  electric  machine  having  a  double-wound  armature  and 
two  commutators.  One  winding  with  its  commutator  receives 
the  high  line  voltage  and  operates  as  a  motor.  The  other 
winding  and  its  commutator  furnishes,  as  a  dynamo,  low 
tension  current.  The  field  is  common  to  both  ^vindings.  Fig. 
37  shows  a  small  machine  of  this  kind,  adapted  to  receive  5,000 
volts  from  the  line,  and  to  deliver  110  volts,  or  vice  versa. 

This  particular  machine  works  at  constant  voltage  on  both 
circuits.  Either  circuit,  however,  could  be  made  to  work  at 
constant  current,  provided  the  means  of  regulation  for  this 
purpose  were  so  chosen  as  to  leave  the  field  and  speed  un- 
changed. 

The  cost  of  a  motor  generator,  while  less  than  that  of  two 
separate  machines,  is  still  high,  and  although  its  efficiency  is 
somewhat  greater  than  that  of  the  pair  mentioned  above,  it  is 
obtained  at  the  cost  of  a  rather  complicated  armature,  which, 
from  a  practical  standpoint,  is  quite  objectionable. 

In  spite  of  the  difficulties  incident  to  working  incandescent 
lamps  from  a  high,  voltage  constant  current  circuit,  the  ease 
with  which  such  circuits  can  be  worked,  even  if  for  power 
alone,  at  voltages  far  above  those  available  on  the  constant 
potential  system,  encouraged  their  installation  during  the 
period  between  the  first  efforts  at  long  distance  transmission 
and  the  more  recent  date  at  which  alternating  current  appa- 
ratus has  become  thoroughly  available.  For  some  years  it 
was  constant  current  or  nothing,  so  far  as  long  distance  trans- 
mission, coupled  with  distribution,  was  concerned. 

As  a  result  of  the  various  adverse  conditions  mentioned, 
transmission  at  constant  current  has  never  made  really  any 
headway  in  American  practice,  and  in  fact  the  method  has  been 
followed  to  a  noticeable  extent  in  only  one  locality  —  San  Fran- 
cisco. There,  through  the  activity  of  local  exploiters,  constant 
current  power  circuits  were  early  established  and  remained 
in  fairly  successful  operation  for  several  years. 


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104  ELECTRIC   TRANSMISSION  OF  POWER. 

There  were  until  recently  three  companies  op)erating  con- 
stant current  circuits  in  San  Francisco  for  the  distribution 
of  power.  The  currents  empoyed  were  of  10,  15,  and  20  am- 
peres. Most  of  the  motors  were  small,  a  very  large  propor- 
tion of  them  being  under  one  horse-power.  The  total  num- 
ber of  motors  in  circuit  on  the  various  systems  was  between 
six  and  seven  hundred. 

Except  in  San  Francisco,  what  few  constant  current  motors 
have  been  in  operation  were  operated  on  regular  arc  circuits. 
Their  use  has  been  much  discouraged  by  the  operating  com- 
panies, and  very  few  such  motors  are  now  manufactured  or 
sold ;  in  fact,  constant  current  distribution  in  modem  American 
practice  is  almost  non-existent.  Abroad,  the  conditions  are 
somewhat  different,  and  on  the  Continent  constant  current 
distribution  for  long  distance  transmission  work  has  been  ex- 
ploited to  a  very  considerable  extent,  probably  owing  to  the 
early  and  successful  establishment  of  a  number  of  transmission 
plants  for  single  motors  worked  on  the  series  system.  There 
are  several  successful  plants  operated  at  constant  current,  one 
of  the  most  considerable  of  them  being  that  at  Genoa,  which 
is  an  excellent  example  of  the  kind  and  as  such  is  worth  more 
than  a  passing  mention,  even  although  the  probability  is  that  it 
will  seldom  be  duplicated,  at  least  on  this  side  of  the  Atlantic. 

The  Genoa  transmission  is  derived  from  the  River  Gorzente, 
which  about  twenty  years  ago  was  developed  for  hydraulic  pur- 
poses, artificial  lakes  being  established  and  a  tunnel  about  IJ 
miles  long  being  built  for  an  outlet.  Beyond  the  tunnel,  an 
aqueduct  some  fifteen  miles  in  length  conveyed  the  water  to 
Genoa,  where  a  considerable  amount  of  power  is  utilized 
directly.  In  this  development  there  was  left  at  the  mouth  of 
the  tunnel  an  unused  fall  of  nearly  1,200  feet  aside  from  the 
head  employed  in  the  aqueduct.  This  has  been  developed 
electrically.  It  was  divided  into  three  partial  falls  of  338,357, 
and  488  feet,  respectively.  At  each  of  these  was  erected  a 
generating  station  with  its  own  transmission  line.  These 
stations  were  named  after  the  three  renowned  electricians, 
Galvani,  Volta,  and  Pacinotti.  The  first  mentioned  station 
was  the  first  installed.  It  consists  of  two  generators  operated 
in  series.     Each  is  of  about  50  KW  capacity,  giving  47  amperes 


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TRANSMISSION  BY  CONTINUOUS  CURRENTS.        105 


T-i 


g^iJ 


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106  ELECTRIC  TRANSMISSION  OF  POWER, 

with  a  maximum  pressure  of  1,100  volts.  Current  is  kept  con- 
stant by  regulating  by  hand  the  speed  of  the  dynamos, 
through  the  gate  which  controls  the  turbines.  Each  dynamo 
is  provided  with  an  automatic  switch,  short-  circuiting  the  ma- 
chine in  case  of  extreme  rise  in  voltage.  This  Galvani  station 
was  a  preliminary  or  experimental  station,  and  was  followed  up 
by  the  establishment  of  two  others  which  supply  the  power  to 
Genoa.  One  of  these  stations,  which  is  thoroughly  typical  of 
the  system  employed,  is  shown  in  Fig.  38.  It  consists  of  four 
turbines,  each  driving  a  pair  of  dynamos  of  a  little  less  than 
50  KW  output  at  45  amperes  and  about  1,000  volts. 

These  dynamos  are  similar  to  those  in  the  Galvani  station, 
but  the  regulation  for  constant  current  is  obtained  in  a  dif- 
ferent manner.  The  dynamos  are  separately  excited,  the 
fields  being  supplied  in  parallel  from  a  small  dynamo  driven 
by  a  separate  waterwheel.  The  speed  of  this  exciter  is  auto- 
matically varied  by  controlling  its  turbine  in  response  to 
changes  of  current  in  the  circuit.  All  the  dynamos  are  oper- 
ated in  series,  and  like  those  in  the  Galvani  station  are  direct 
coupled  in  pairs.  The  machines  are  insulated  with  enormous 
care,  heavy  layers  of  mica  being  placed  between  the  magnets 
and  the  bed  plates,  while  the  windings  themselves  are  very 
elaborately  protected.  Carbon  brushes  are  employed,  and 
the  commutators  are  reported  to  behave  admirably.  Each 
dynamo  is  protected  by  most  elaborate  safety  devices,  as  in  the 
Galvani  station.  The  regulation  is  said  to  be  excellent,  even 
under  considerable  changes  of  load. 

The  third  station,  Pacinotti,  contains  eight  machines  of  the 
same  capacity  as  those  in  the  preceding.  They  are  governed 
as  in  the  Galvani  station  by  controlling  their  speed.  This, 
however,  is  done  by  an  electrical  motor-governor  controlled 
by  a  relay  on  the  main  line  and  worknig  in  one  direction  or 
the  other  as  the  occasion  may  recjuire.  In  all  the  stations  are 
carried  out  the  same  thorough  precautions  regarding  insula- 
tion, and  each  machine  has  around  it  an  insulated  floor  sup- 
ported on  porcelain.  The  line  voltage  from  each  of  the  two 
stations  last  mentioned  is  from  6,000  to  8,000,  and  two 
circuits  are  carried  into  Genoa,  the  extreme  distance  of  trans- 
mission being  about  eighteen  miles.    The  conductors,  of  wire 


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TRANSMISSION  BY  CONTINUOUS  CURRENTS.        107 

9  mm.  in  diameter  (nearly  No.  00  B.  &  S.),  are  for  the  most 
part  bare,  except  when  passing  through  villages,  and  are  sup- 
ported on  oil  insulators  carried  by  wooden  poles,  save  in  some 
few  cases  where  iron  poles  have  been  used.  The  two  circuits 
running  into  Genoa  transmit  power  for  motors  only.  The  loss 
in  the  line  at  full  load  is  8  per  cent. 

At  full  load,  nearly  1,000  HP  is  transmitted  over  the  lines, 
the  motors  being  of  all  sizes  between  10  and  120  horse-power. 
They  are  of  the  ordinary  series-wound  type,  and  their  speed  is 
automatically  controlled  by  centrifugal  governors,  which  act 
by  varying  the  field  strength.  Fig.  39  shows  one  of  the  Genoa 
motors,  fitted  with  an  automatic  governor  acting  upon  a  com- 


FlO.  39. 

mutated    field    winding.     They    are    provided    with    carbon 
brushes,  and  are  reported  to  operate  very  successfully. 

It  is  to  be  noted,  however,  that  the  motors  are  placed  in 
special  rooms  with  insulated  floors  and  walls,  owing  to  the 
enormous  voltage  which  has  to  be  taken  into  the  buildings. 
They  are  fitted  with  heavy  fly-wheels  to  assist  the  governors, 
and  with  automatic  switches  to  short  circuit  around  the  motor 
in  case  of  excessive  voltage.  The  motors  are  under  the 
special  care  of  skilled  assistants  connected  with  the  staff  of 
the  generating  station,  who  inspect  the  lines  and  go  over  the 
motors  at  intervals  of  a  few  days.  These  extraordinary  pre- 
cautions both  in  the  matter  of  insulation  and  skilled  attendance 
account    in  great  measure  for  the  success  of  what,   under 


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108  ELECTRIC  TRANSMISSION  OF  POWER. 

American  conditions,  would  have  almost  infallibly  resulted  in 
disastrous  failure.  The  efficiency  of  the  plant  from  turbine 
shaft  to  motor  pulleys  is  said  to  be  a  little  over  70  per  cent. 

As  may  be  judged  from  this  description,  the  whole  instal- 
lation is  of  enormously  complicated  character,  although  per- 
haps as  simple  and  efficient  as  any  alternating  plant  of  the 
same  early  date.  The  plan  of  the  Volta  station  for  the  most 
part  explains  itself.  The  switchboards  for  each  machine  with 
their  plugs  for  connecting  the  pair  of  dynamos  coupled  to  it  are 
shown  at  -4,  dynamos  at  fi,  the  exciters  at  C,  exciter  switch- 
board and  rheostat  at  D,  and  the  solenoids,  which  control  the 
exciter  turbines,  at  E.  Lightning  arresters  are  shown  at  F. 
These  consist  of  a  spark  gap,  impedance  coils  in  series  with 
the  line  and  condenser  shunted  around  them.  Every  motor  is 
provided  with  a  similar  lightning  arrester.  Taken  altogether, 
this  Genoa  plant  is  an  excellent  example  of  the  constant  cur- 
rent system  followed  to  its  legitimate  conclusion.  A  descrip- 
tion of  the  system  is  a  sufficiently  condemnatory  criticism 
judged  from  our  present  point  of  view;  at  the  same  time,  it 
should  be  remembered  that  while  this  station  was  being  built, 
the  method  adopted  was  practically  as  good  as  any  available 
in  the  existing  state  of  the  art,  and  that  the  system  has  in 
more  recent  installations  been  materially  improved.  En- 
couraged by  the  favorable  results  obtained  at  Genoa,  a  similar 
station  was  soon  afterwards  built  delivering  a  maximum  of 
700  HP  at  Brescia  at  an  extreme  full-load  pressure  of  about 
15,000  volts  over  a  12-mile  line.  Since  these  stations  nearly 
a  dozen  others  have  been  installed,  aggregating  about  17,000 
HP,  and  their  performance  has  been  uniformly  good.  In 
spite  of  a  predilection  for  modem  polyphase  work  one  must 
admit  that  a  system  which  has  been  installed  to  such  an 
extent,  and  of  late  in  competition  with  alternating  methods, 
is  far  from  moribund.  Two  strong  points  it  undoubtedly  has; 
freedom  from  all  inductive  disturbances,  and  the  property  of 
carrying  its  extreme  voltage  only  at  full  load,  the  importance 
of  which  will  be  discussed  later.  It  has  shown  itself  capable 
of  doing  steady  and  efficient  work  over  long  distances  and 
under  cHmatic  conditions  by  no  means  favorable.  The  Con- 
tinental makers  of  this  class  of  machinery  have  gone  far  be- 


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I'LATE    III. 


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TRANSMISSION  BY  CONTINUOUS  CURRENTS.       109 

yond  anything  that  has  been  attempted  in  American  practice 
and  have  turned  out  constant  current  dynamos  of  really 
remarkable  properties. 

At  present,  machines  of  50  to  60  amperes  have  been  given 
successfully  E.  M.  F.  as  high  as  3,500  volts,  while  those  of 
100  to  150  amperes  have  gone  to  2,500  volts.  As  they  are 
usually  coupled  in  pairs,  a  single  unit  may  have  a  capacity  of 
about  700  KW,  each  component  machine  giving  over  300^ 
Without  pushing  beyond  present  apparatus  it  then  becomes 
possible  to  arrange  a  plant  of  1,000  to  1,500  KW  having  a 
working  E.  M.  F.  at  full  load  of  10,000  to  14,000  volts.  Such 
a  plant  is  not  especially  complicated  and  is  nearly  as  easy  to 
operate  as  an  alternating  plant.  For  a  load  of  a  few  large 
motors,  it  is  capable  of  good  work,  without,  however,  present- 
ing any  advantages  over  a  polyphase  system  save  that  the 
line  is  simpler  and  the  insulation  requirements  less  severe. 
An  alternating  power  station  of  similar  output  would  contain 
practically  as  many  generators,  for  sake  of  security.  When 
it  comes  to  combined  lighting  and  power  service  the  constant 
current  system  is  hard  pushed.  In  practice,  recourse  is  had 
to  motor  generators.  Perhaps  the  best  idea  of  the  situation 
may  be  given  by  a  brief  description  of  the  Swiss  transmission 
from  Combe-Garot  to  La  Chaux-de-Fonds,  a  distance  of  32 
miles.  At  the  former  place  are  installed  8  generating  units 
each  giving  150  amperes  at  1,800  volts,  giving  a  total  capacity 
of  2,160  KW  at  14,400  volts.  These  generators  are  six  pole 
Thury  machines  with  drum  armatures,  and  are  series  wound. 
Regulation  is  by  automatic  variation  of  the  speed  of  the  tur- 
bines, the  normal  full  load  speed  being  300  r.  p.  m.  The  line 
is  overhead,  of  cables  having  a  cross  section  of  about  300,000 
cm.,  bare  except  in  the  towns  where  the  power  is  delivered  — 
Lode,  and  La  Chaux-de-Fonds  at  the  end  of  the  line.  Motors 
aggregating  2,400  HP  are  in  circuit  at  these  points,  2,000  HP 
being  used  in  the  transforming  stations.  All  motors  above  20 
HP  are  upon  the  high  tension  circuit.  The  substation  at  La 
Chaux-de-Fonds  is  typical  of  the  methods  employed.  It  is 
equipped  with  motor-generators  of  200  HP  working  a  three 
wire  system  at  320  volts  between  the  outside  wires.  One  of 
the  motor-generator  units  is  shown  in  Plate  III.    It  is  composed 


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no 


ELECTRIC  TRANSMISSION  OF  POWER. 


of  two  six  pole  machines  with  a  fly-wheel  between  them.  The 
machine  to  the  left  is  the  motor  and  upon  it  is  mounted  the 
automatic  speed  regulator.  The  principle  upon  which  this 
works  is  shown  in  F\g.  40.  The  regulating  shimt  around  the 
fields  and  the  brush  shifting  mechanism  are  simultaneously 
actuated  by  the  dogs  thrown  into  gear  by  the  governor.  This 
form  of  governor  is  very  generally  used  for. the  motors  upon  the 
system. 

The  efficiency  of  both  generator  and  motor  under  test  has 


Fig.  40. 

been  shown  to  be  93.5  per  cent,  or  87  per  cent  for  the  com- 
plete imit.  Similar  motor-generators  in  connection  with  a 
storage  battery  furnish  current  at  550  volts  for  railway  service. 
Now  the  drop  in  the  line  at  full  load  is  6  per  cent,  so  that 
w^e  are  in  position  to  make  a  very  close  estimate  of  the  effi- 
ciency of  the  system  from  waterwheel  to  low  tension  mains. 
It  is  obviously 

93.5  X  .94  X  .87  =  76.5  per  cent. 

This  is  a  very  creditable  figure  for  the  total  efficiency,  and  it  is 
worth  while  comparing  it  with  the  results  ordinarily  reached  in 
polyphase  working.  Taking  the  generator  at  94  per  cent,  the 
raising  and  reducing  transformers  at  97.5  per  cent  each,  the 


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TRAXSMISSION  BY  COXTINUOUS  CURREXTS,        111 

line  at  .94,  and  the  distributing  banks  of  transformers  at  96.5, 
we  have 

.94  X  .94  X  .975  x  .975  X  .965  =  .81. 

The  difference  is  substantially  that  due  to  the  difference  in 
efficiency  between  the  static  transformers  and  the  motor  gen- 
erator. If  the  comparison  be  made  w4th  the  railway  part  of 
the  propositiojt,  assuming  the  use  of  a  rotary  converter,  the 
case  would  stand  about  as  follows: 

.94  X  .94  X  .975  x  .975  x  .94  =  .79. 

In  the  simple  case  of  large  motors  the  advantage  lies  rather 
the  other  way,  for  the  constant  current  plant  would  show 

.935  X  .94  X  .935  =  82.1 

against,  for  the  alternating  plant, 

.94  X  .94  X  .975  X  .975  x  .94  =  .79. 

This  merely  indicates  that  after  passing  the  voltages  which 
can  be  derived  directly  from  the  armature,  more  is  lost  in  the 
transformers  than  is  gained  in  a  low  voltage  winding.  The 
figures  for  the  efficiency  of  the  alternating  plant  are  taken 
from  actual  data  on  machines  of  about  the  capacity  concerned. 
To  tell  the  unvarnished  truth,  a  constant  current  transmission 
coupled  with  a  three  wire  distribution  at  220  to  250  volts  on 
a  side  is  capable  of  giving  even  the  best  alternating  system  a 
pretty  hard  rub  over  moderate  distances.  In  this  coimtry 
no  constant  current  power  transmission  machinery  is  avail- 
able, but  where  it  is  readily  to  be  had,  it  is  by  no  means  out  of 
the  game.  A  still  larger  constant  current  transmission  plant 
is  now  in  operation  between  the  falls  of  the  Rhone  at  Saint- 
Maurice,  and  Lausanne,  a  distance  of  34.8  miles.  The  first 
installation  is  of  five  pairs  of  generators  aggregating  3,300 
KW,  giving  at  full  load  a  combined  voltage  of  23,000.  The 
current  in  this  case  is  150  amperes,  as  in  the  Combe-Garot 
plant  just  described. 

POWER   TRANSMISSION   AT   CONSTANT   POTENTIAL. 

The  transmission  of  power  to  series-woimd  motors  at  con- 
stant potential  is  a  branch  of  the  art  which  as  regards  station- 


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112  ELECTRIC  TRANSMISSION  OF  POWER, 

aiy  motors  has  been  developed  only  in  special  cases.  It  is, 
however,  the  method  universally  employed  for  electric  rail- 
way work.  Two  or  three  sporadic  efforts  have  been  made  to 
operate  electric  railway  systems  at  constant  current,  but  with 
such  indifferent  success  that  the  method  has  been  abandoned. 
Coimting  in  electric  railways,  it  is  safe  to  say  that  at  present 
the  majority  of  all  electrical  power  transmission  in  the  world 
is  done  ^\'ith  series  motors  worked  at  constant  potential  or  as 
nearly  constant  potential  as  may  be  practicable.  As  before 
mentioned,  regulation  is  generally  obtained  through  the  use  of 
rheostats  in  series  with  the  motors,  thereby  cutting  down  the 
applied  voltage,  or  by  throwing  the  motors  either  in  parallel 
or  in  series  with  each  other,  or  in  the  third  place  by  a  combi- 
nation of  the  above  methods.  Concerning  the  operation  of 
motors  thus  arranged,  sufficient  has  been  said  to  explain  the 
situation  clearly.  The  general  good  properties  of  the  method 
are  most  prominently  exhibited  in  the  simplicity  of  the  motor 
windings  and  the  very  powerful  effort  which  can  be  obtained 
in  starting  the  motors  from  rest.  These  properties  are  of 
extreme  value  in  railway  service. 

Aside  from  the  operation  of  electric  railways,  series  motors 
at  constant  potential  are  frequently  employed  for  hoists  and 
similar  work  where  a  powerful  starting  torque  and  considerable 
range  of  speed  at  the  will  of  the  operator  are  desirable.  In 
spite  of  the  large  use  of  motors  for  such  purposes,  there  are  no 
plants  either  here  or  abroad  which  may  be  said  to  be  operated 
exclusively  after  this  method,  for  it  is  generally  found  desir- 
able to  combine  in  the  same  system  series-wound  motors  for 
severe  work  and  shunt-wound  motors  for  purposes  where  uni- 
form speed  is  of  prime  importance.  As  a  rule  the  power  trans- 
mission so  accomplished  is  over  a  comparatively  small  distance 
and  really  involves  the  problem  of  distribution  more  than 
transmission  alone.  A  very  large  number  of  electric  hoists 
designed  by  different  makers  are  in  use  at  various  points 
throughout  this  and  other  countries,  doing  service  in  mines, 
operating  elevators  of  one  kind  or  another,  working  derricks,  and 
travelling  cranes  and  employed  for  a  large  variety  of  similar 
purposes.  Many  of  the  motors  employed  are  of  the  ordinary 
railway  type. 


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TRAXSAflSSIOX  BY  COXTIXUOUS  CURREXTS.        113 

The  voltage  utilized  for  this  work,  in  America  at  least,  is 
usually  either  200  to  250  volts  or  500  to  600  volts,  the  former 
being  most  generally  used  in  mines,  where  difficulties  of  insu- 
lation are  considerable,  or  in  operating  motors  supplied  by 
three- wire  systems  already  installed.  The  latter  voltage  is  gen- 
erally selected  for  work  above  ground.  None  of  the  plants  so 
equipped  are,  however,  sufficiently  large  or  characteristic  to  be 
worth  a  detailed  description.  The  power  installations  and  the 
methods  of  distribution  are  in  general,  closely  similar  t«  those 
employed  for  electric  railway  work.  Plants  of  higher  vol- 
tage than  from  500  to  600  are  so  infrecjuent  as  to  be  hardly 
worth  considering  in  practical  engineering.  It  is  perfectly 
possible  to  wind  series  motors  for  voltages  considerably  ex- 
ceeding this  figure,  say  for  1,000  or  1,200  volts,  or.  in  rare 
cases,  more,  provided  the  units  arc  of  tolerable  size,  but  inas- 
much as  most  plants  for  the  distribution  of  power  require  both 
large  and  small  motors,  w^ound  both  series  and  shunt,  the 
general  voltage  is  in  nearly  every  case  kept  at  a  point  at  which 
it  will  be  easy  to  meet  these  varied  reciuirements;  therefore 
500  volts,  the  American  standard  for  railway  practice,  has 
usually  been  selected. 

The  only  noteworthy  exception  may  be  found  in  the  use 
of  the  Edison  three-wire  system  for  distribution  of  power 
to  railway  and  other  motors.  By  this  method  it  become}? 
possible  to  transmit  the  power  at  the  virtual  voltage  of  1,000 
and  to  employ  1,000  volt  motors,  either  series-  or  shunt- wound 
for  the  larger  units  in  order  to  help  in  preserving  the  general 
balance,  while  at  the  same  time  using  motors  of  all  sizes  with 
any  kind  of  direct  current  winding,  connected  between  the 
middle  wire  and  either  of  the  outside  wires.  The  advantages 
of  such  an  arrangement  are  very  evident,  and  if  the  number 
of  motors  be  considerable,  so  that  it  is  possible  to  balance  the 
system  with  a  fair  degree  of  accuracy,  we  have  at  our  disposal 
a  very  convenient  niethod  for  the  distribution  of  continuous 
currents.  It  is  interesting  to  note  that  this  scheme  foimd  its 
first  considerable  development  in  electric  railway  service  itself. 
Of  course,  the  use  of  both  110  and  220  volt  motors  on  Edison 
three-wire  systems  is  very  common,  but  the  extension  of  the 
plan  to  operating  electric  roads,  and  under  conditions  which 


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114  ELECTRIC   TRANSMISSION  OF  POWER. 

as  regards  balance  are  somewhat   trying,   is  a   considerable 
step  toward  an  individual  method. 

The  method  of  working  electric  railways  on  the  three-wire 
system  is  well  shown  in  Fig.  41.  Here  the  road  is  a  double 
track  one,  to  which  the  method  is  generally  best  suited.  The 
ground,  track,  and  supplementary  wires  serve  as  a  neutral 
wire,  both  tracks  being  placed  in  parallel  for  this  purpose 
and  thoroughly  bonded.  On  a  double  track  road,  the  cars 
running  on  each  side  of  the  system  will  be  substantially  the 
same  in  number,  and  if  the  total  number  of  cars  be  consider- 
able, a  very  fair  balance  can  be  obtained,  although  never  as 
good  as  is  customarily  and  necessarily  used  in  an  Edison  three- 
wire  system  for' lighting.  In  order  to  still  further  improve 
the  balance  of  the  system  and  prevent  its  being  disturbed  as 


Fig.  41. 

might  otherwise  occur  by  a  blockade  on  one  track  at  some 
point,  it  is  better  to  make  the  trolley  wire  above  each  track 
consist  of  sections  of  alternate  polarity  and  of  convenient 
length,  so  that  even  in  case  of  a  bk)ckade,  stopping  a  con- 
siderable number  of  cars,  the  load  would  be  removed  almost 
equally  from  both  sides  of  the  system. 

Installed  in  this  way,  a  railway  system  is  operated  at  a  virtual 
voltage  of  1,000,  and  the  saving  of  copper  over  the  ordinary 
distribution  at  500  volts  is  considerable,  in  spite  of  the  inevitable 
lack  of  balance  and  the  loss  of  the  track  as  part  of  the  main 
conducting  system.  In  one  large  French  plant  electric  loco- 
motives are  used,  each  utilizing  both  sides  of  the  three- wire 
system  so  as  to  preserve  a  complete  balance.  Nevertheless,  the 
three-wire  distribution  for  trainways  has  proved  generally 
imsatisfactory  on  account  of  the  complication  of  the  overhead 


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TRANSAflSSION  BY  COXTiyUOUS  CURRENTS,        115 

wiring  and  the  difficulty  of  preserving  balance,  so  that  it  has 
entirely  disappeared  from  American  practice.  A  few  years 
ago  it  was  tried  in  Bangor,  Maine  ;  Portland,  Oregon ;  and 
experimentally  elsewhere,  but  was  abandoned  after  no  very 
long  experience  as  troublesome  and  unreliable.  It  has  found 
some  use  abroad,  even  in  a  few  recent  plants,  and  is  reported 
to  be  successful  in  spite  of  the  difficulties  here  mentioned. 
The  increase  in  voltage  obtained  by  its  use  is  not  sufficient 
to  answer  the  general  purposes  of  interurban  service,  and  the 
ease  with  which  power  can  be  transmitted  by  altematuig 
currents  and  utilized  through  rotary  converters,  or  even  directly 
upon  the  cars,  has  obviated  the  necessity  for  so  dubious  a 
method  of  obtaining  higher  economy  of  transmission. 

INTERDEPENDENT    DYNAMOS    AND    MOTORS. 

Aside  from  the  distribution  of  power  for  railway  purposes, 
by  far  the  most  interesting  kind  of  power  transmission  by 
continuous  currents  is  that  in  which  a  special  combination  of 
two  series  machines  is  employed,  giving  a  self-regulating 
system  comprising  a  motor  unit  and  a  generator  unit.  This 
plan  has  been  successfully  used  abroad,  but  has  never  been 
employed  in  American  engineering  practice  except  in  an 
experimental  and  tentative  way,  owing  largely  to  the  difficulties 
that  have  been  encountered  in  the  production  of  large  direct- 
current  generators  for  high  voltage. 

While  it  is  not  at  all  a  difficult  matter  to  build  a  machine 
giving  five  or  six  thousand  volts  with  a  rather  small  current, 
such  as  is  used  in  arc  lighting,  the  troubles  at  the  commuta- 
tor have  proved  forbidding  when  any  attempt  has  been  made 
to  use  currents  large  enough  to  obtain  units  of  any  consider- 
able size.  Power  transmission  in  this  country  took  its  first 
stimulus  from  the  development  of  polyphase  apparatus  and 
methods,  and  consequently,  so  far  as  the  art  has  now  been 
carried  forward,  it  has  been  almost  wholly  in  the  line  of  alter- 
nating current  work.  It  is  obvious,  nevertheless,  that  a 
system  of  power  transmission  such  as  we  are  considering, 
possesses  great  convenience  where  single  units  are  to  be  operated 
over  moderately  long  distances.     In  the  first  place,  the  induc- 


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116  ELECTRIC  TRANSMISSION  OF  POWER. 

tive  difficulties  familair  with  alternating  currents  are  avoided. 
In  the  second  place,  the  motors  are  self-starting  imder  load, 
a  condition  that  was  not  true  of  alternating  machinery  prior 
to  the  introduction  of  the  polyphase  system.  Through  the 
energy  of  several  foreign  engineers,  notably  Mr.  C.  E.  L.  Brown, 
much  was  done  in  power  transmission  by  this  method  long 
before  alternating  current  apparatus  had  been  suitably  devel- 
oped. The  same  difficulties  were  encountered  abroad  as 
here.  It  proved  to  be  very  difficult  to  build  machines  of  suffi- 
cient voltage  and  of  any  considerable  output. 

In  this  connection  it  is  noteworthy  that  nearly  all  the  plants 
of  this  character  on  the  Continent  have  been  installed  at  rela- 
tively low  voltages,  most  of  thein  less  than  1,000,  correspond- 
ing in  general  character  to  the  American  plants  over  similar 
distances  worked  at  constant  potential.  In  the  very  few 
instances  where  long  distances  have  been  attempted,  the 
usual  method  has  been  to  employ  generators  and  motors  per- 
manently connected  together  in  series,  on  account  of  the 
impracticability  of  getting  sufficient  power  in  one  unit  at  a 
very  high  voltage.  This  proceeding  somewhat  complicates 
the  system.  In  addition,  the  generators  and  motors  have  to 
be  especially  designed  for  each  other  in  order  to  secure  regu- 
lation; which,  of  itself,  is  a  considerable  disadvantage. 

This  last  difficulty  may  be  in  part  avoided  by  using  a  shunt 
around  the  field  coils  of  the  generator,  thereby  changing  its 
regulation  under  variations  of  current.  A  similar  device  is 
widely  used  in  this  country  in  connection  with  compoimd- 
wound  generators,  where  a  shunt  applied  across  the  terminals 
of  the  series  coils  is  used  to  regulate  the  compounding.  In 
either  case,  the  obvious  result  of  such  a  shunt  is  to  diminish 
the  change  in  the  field  produced  by  a  given  increase  in  cur- 
rent. In  this  way  the  necessity  for  special*  machines  can  be 
partially  obviated.  The  plants  installed  on  this  peculiar  series 
plan  have  been  imiformly  successful,  and  permit  of  the  con- 
venient transmission  of  moderate  amounts  of  power  over  con- 
siderable distances.  Such  plants  have  even  been  employed 
in  connection  with  motor-generators  to  supply  a  general 
distribution  system,  though  evidently  at  a  high  cost  for  appa- 
ratus. 


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TRANSMISSION  BY  CONTINUOUS  CURRENTS.       117 

In  order  to  distribute  low  tension  currents  from  such  a 
transmission  system,  it  is  necessary  to  employ  either  a  motor, 
coupled  to  a  dynamo,  or  a  composite  machine  with  a  double 
winding,  as  in  case  of  transmission  at  constant  current.  Either 
alternative  involves  the  loss  of  energy  substantially  equiva- 
lent to  that  lost  in  two  dynamo-electric  machines  of  the 
capacities  concerned.  These  losses  arc  necessarily  much  more 
serious  than  those  in  an  alternating  current  transformer,  as 
has  already  been  seen.  They  are  likely  to  amoimt  to  from  12 
to  15  per  cent,  so  that  quite  aside  from  the  efficiency  of  the 
generating  dynamo  and  of  the  line,  the  price  paid  for  the  privi- 
lege of  obtaining  a  low  tension  current  is  considerable. 

For  the  dehvery  of  power  alone,  where  motors  in  series 
coupled  to  appro j)riate  generators  can  be  used,  the  method 
is  well  fitted  for  use  under  certain  circumstances  and  is  closely 
approximate  in  efficiency  to  that  which  would  be  obtained 
by  an  alternating  current  transmission  over  the  same  dis- 
tance. It  is  worth  mentioning  that  one  of  the  longest  of  the 
early  power  transmissions  was  operated  upon  this  now  obsoles- 
cent system. 

The  plant  in  question  is  that  which  was  installed  nearly 
fifteen  years  ago  for  operating  the  Biberest  Paper  Mills,  near 
Soluere,  Switzerland.  The  power  is  derived  from  the  River 
Suze,  near  Bienne,  and  the  distance  of  transmission  is  a  little 
less  than  twenty  miles.  At  the  generating  station  the  avail- 
able head  of  water  is  about  forty-five  feet,  and  the  quantity 
is  sufficient  to  generate  about  400  HP.  The  power  station 
contains  a  400-HP  turbine  running  at  120  revs,  per  minute, 
of  which  the  vertical  shaft  is  connected  by  means  of  beveled 
gear  to  two  130-KW  dynamos.  They  are  six  pole  machines. 
Gramme  wound,  and  give  at  275  revs,  per  minute  about  40 
amperes  at  3,300  volts.  The  two  machines  are  connected 
in  series,  giving  a  working  potential  of  6,600  volts  on  the  line. 
It  should  be  noted  that  great  care  is  taken  in  insulating  them, 
the  bed  plates  being  carried  on  porcelain  insulators.  Carbon 
brushes  are  employed.  The  line  is  a  bare  copper  wire,  7 
millimetres  in  diameter,  about  No.  1  B.  &  S.  gauge.  The 
line  runs  through  a  mountainous  country,  and  is  Uberally 
provided  vrith  lightning  arresters  at  various  points. 


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118 


ELECTRIC   TRANSMISSION  OF  POWER. 


The  two  motors  at  the  mills  are  duplicates  of  the  gener- 
ators, the  only  modifications  being  those  to  insure  their  self- 
regulation.  They  run  at  200  revs,  per  minute  on  6,000  volts 
delivered,  and  give  about  155  HP  each.  The  commercial 
efficiency  of  this  interesting  system  is  somewhat  in  excess  of 
75  per  cent  at  full  load.  Fig.  42  shows  one  of  the  motors  on 
its  foundation,  and  its  coupling  to  its  mate.  This  plant  con- 
stitutes  the   most  striking  example  of  long  distance  trans- 


Fio.  42. 

mission  by  series-wound  interdependent  generators  and  motors, 
and  probably  exhibits  the  system  at  its  best. 

In  this  country  the  system  has  not  been  used  in  anything 
more  than  an  experimental  way,  owing  principally  to  two 
reasons:  first,  for  short  distances,  involving  not  more  than 
1,000  volts,  shunt-wound  generators  and  motors  working  on 
the  two-wire  or  three-wire  systems  afford  better  opportmiity 
for  distribution,  inasmuch  as  their  use  is  not  limited  to  single 
mechanical  units;  second,  no  serious  demand  for  long  distance 
transmission  arose  in  America  prior  to  the  development  of 
the  alternating  system  to  the  point  at  which  alternating 
motors  became  thoroughly  practicable.  It  has  been  charac- 
teristic of  American  electrical  engineering  that  it  has  occupied 


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TRANSMISSION  BY  CONTINUOUS  CURRENTS.  119 
« 
itself  with  one  thing  at  a  time.  The  development  of  the 
electric  light  was  followed  by  a  concentration  of  energy  on  the 
electric  railroad,  and  this  has  of  late  years  been  succeeded  by 
extensive  power  transmission  enterprises,  often  in  themselves 
involving  railway  work.  Such  a  mental  habit  is  not  conducive 
to  an  even  development,  but  probably  accomplishes  quite  as 
much  as  a  more  symmetrical  advance. 

CONSTANT   POTENTIAL   SYSTEMS. 

Shunt-  or  compound-woimd  generators  used  in  connection 
with  shunt-wound  motors  have  found  very  extensive  use 
in  this  country,  working  over  inconsiderable  distances.  The 
very  obvious  advantage  of  such  a  system  is  that  it  permits  the 
ready  distribution  of  power  as  well  as  its  easy  transmission. 
If  it  becomes  necessary  to  transmit  power  from  one  point  to 
another,  the  chances  are  nuich  more  than  even  that  at  the 
distributing  end  of  the  Hne  it  will  be  desirable  to  utilize  the 
power  in  a  number  of  units  of  varying  size.  Such  an  arrange- 
ment bars  out  transmission  from  series  dynamos  unless  upon 
the  constant  current  system  with  its  inherent  difficulties  of 
regulation,  whereas  -with  shunt- wound  apparatus  the  problem 
is  easy.  It  often  happens,  as  previously  mentioned,  that  at 
the  receiving  end  both  series  and  shunt  motors  are  used,  the 
former  for  hoisting  and  similar  work,  the  latter  for  operation 
at  constant  speeil. 

The  growth  of  the  electric  railway  has  encouraged  the  estab- 
lishment of  such  transmission  plants,  and  their  number  is  very 
considerable,  scattered  over  all  parts  of  the  Union,  not  a  few 
of  them  being  in  the  mining  regions  of  the  Rocky  Mountains 
and  on  the  Pacific  coast,  as  well  as  in  various  isolated  plants 
through  the  rest  of  the  country.  In  most  of  them,  the  dis- 
tances being  moderate,  an  initial  voltage  of  from  500  to  600 
has  been  employed;  more  rarely,  voltages  ranging  from  1,000 
to  1,800.  Plants  of  these  latter  voltages  have  now  generally 
been  replaced  by  polyphase  machines.  The  efficiency  of  this 
method  of  transmission  is  about  the  same  as  that  of  the  series 
method,  just  described,  but  ^ith  the  advantage  that  the 
shunt  motor  supplied  at  constant  potential  can  advantageously 


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120  ELECT HIC   TRAXSMhSSION  OF  POWER. 

be  (listri})uted  wherever  the  work  is  to  he  done,  while  with  inter- 
dependent series  units  any  distribution  of  power  has  to  be  ac- 
compHshed  by  means  of  shafting  and  belting  or  its  equivalent. 

The  net  efficiency  from  generator  to  driven  machine  is  likely 
to  be  rather  better  with  the  transmission  at  constant  potential 
than  in  the  case  just  discussed.  The  generators  and  motors 
are  of  nearly  the  same  efficiency;  the  line  at  ordinary  dis- 
tances is  customarily  worked  at  about  the  same  pressure  in' 
both  methods,  but  distribution  by  shafting  is  far  less  efficient 
at  any  but  short  distances  than  distri})ution  of  electric  power 
by  wire.  The  loss  from  the  centre  of  distribution  to  individual 
motors  will  very  seldom  exceed  5  per  cent,  w^hile  the  loss  in 
equivalent  shafting  will  seldom  be  less  than  10  per  cent,  and 
more  often  20  or  more;  in  fact,  it  generally  turns  out  upon 
investigation  that  so  far  as  efficiency  is  concerned  there  is 
a  noticeable  saving  in  transmitting  power  electrically,  even 
within  the  limits  of  a  mill  or  large  factory,  over  the  results 
which  can  be  obtained  by  the  use  of  transmission  by  shafts 
and  belts.  In  a  large  building  where  the  power  is  to  be  widely 
distributed,  it  seldom  happens  that  the  loss  in  the  shafting  is 
less  than  25  per  cent.  Anything  in  excess  of  this  figure  repre- 
sents remarkably  good  practice.  With  motors,  80  per  cent 
efficiency,  if  the  units  are  of  tolerable  size,  can  be  reached 
without  much  difficulty,  and  there  are  comparatively  few 
cases  where  the  efficiency  need  fall  lower  than  75.  In  such  a 
plant,  installed  some  years  since  in  a  Belgian  gun  factory, 
and  described  in  the  last  chapter,  the  guaranteed  efficiency 
was  76.6  per  cent.  As  the  efficiency  of  the  dynamo  was 
reckoned  at  but  90  per  cent,  the  total  efficiency  would  in 
practice  be  raised  without  difficulty  to  78  or  79  per  cent  at  full 
load. 

As  regards  efficiency  in  general,  aside  from  the  disadvan- 
tages previously  mentioned  in  changing  the  voltage  of  direct 
current  circuits,  the  efficiency  of  transmission  by  such  currents 
is  in  itself  as  high  as  has  ever  been  reached  by  other  means. 
There  is  no  material  difference  between  the  efficiency  of  direct 
and  alternating  current  generators,  nor  between  the  efficiency 
of  direct  current  motors  and  the  polyphase  motors,  at  least, 
p,mong  alternating  motors.     In  these  particulars,  the  direct 


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TRASSMISSIOy  BY  CONTINUOUS  CURRENTS.         121 

current  is  able  to  hold  its  own  against  all  coniors,  and  in  the 
cost  of  motors  it  has  at  present  a  material  advantage. 

As  regards  transmission  of  power  over  considerable  dis- 
tances, a  case  has  already  been  mentioned  in  which  the  result 
is  as  good  as  can  reMS()na])ly  be  expected.  Direct  current, 
however,  continually  rujis  into  the  limitati(m  of  available  vol- 
tage as  sixm  as  distribution  is  to  be  attempted.  Where  single 
or  a  few  large  motor  units  are  to  be  used,  consisting  of  either 
single  machines  or  groups  operated  as  a  unit,  the  efficiency  of 
the  system  is  likely  to  be  as  high  as  that  obtained  from  units 
of  similar  magnitude  on  alternating  current  systems.  The 
only  disadvantage  of  the  direct  current  in  point  of  efficiency 
in  this  particular  case  is  that  if  the  amount  of  power  to  be 
transmitted  be  large,  it  is  necessary  to  use  generators  and 
motors  coupled  in  series,  while  if  alternating  currents  were 
used,  one  would  have  the  advantage  of  employing  a  single 
machine  of  equivalent  capacity.  The  principal  disadvantage 
of  direct  current  machinery  is  the  commutator,  which  at  high 
voltages  is  likely  to  be  sooner  or  later  the  source  of  consider- 
able trouble.  Careful  mechanical  and  electrical  construction 
may  materially  reduce  this  difficulty,  but  it  always  remains  to 
be  faced,  and  is  liable  at  any  time  to  become  troublesome. 

On  long  lines,  the  direct  current  has  the  advantage  of  pro- 
ducing no  inductance  in  the  line,  an  advantage,  however, 
which  does  not  apply  to  plants  which  can  advantageously  be 
operated  as  single  units.  Such  a  single  vmit  system,  arranged 
for  alternating  currents,  can  have  the  inductance  of  the  cir- 
cuit completely  nullified  by  the  simple  expeclient  of  strength- 
ening the  field  of  the  motor. 

It  must  be  remembered,  however,  that  in  several  particulars 
continuous  current  has  peculiar  advantages.  In  the  first 
place,  it  is  well  known  that  a  direct  current  is  decidedly  less 
dangerous  than  an  alternating  current  of  the  same  nominal 
voltage,  so  far  as  the  question  of  life  is  concerned.  The  differ- 
ence between  the  two  is  even  greater  than  would  be  indicated 
by  the  difference  in  maximum  voltage. 

An  alternating  current  has  a  maximum  voltage  of  approxi- 
mately 1.4  times  its  mean  effective  voltage,  and  in  addition  to 
this   an    alternating    current    is    certainly   intrinsically    more 


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122  ELECTRIC  TRANSMISSION  OF  POWER. 

dangerous  by  reason  of  the  greater  shock  to  the  nervous 
system  producec^  by  the  alternations  of  E.  M.  F.  The  ease 
of  tranforming  alternating  current  to  a  lower  voltage  partially 
obviates  this  objection,  but  the  fact  remains.  So  far  as 
danger  of  fire  is  concerned,  the  continuous  current  has  the 
power  of  maintaining  a  much  more  formidable  arc  than  an 
alternating  current  of  the  same  effective  voltage;  but,  on  the 
other  hand,  the  alternating  current  has  somewhat  greater 
maximum  voltage  with  which  to  start  the  arc,  so  that,  practi- 
cally, honors  are  even. 

The  increase  of  experience  with  resonance  and  kindred 
phenomena,  acquired  on  long  lines  and  at  high  voltages,  has 
emphasized  the  fact  that  alternating  transmission  work  is  not 
exactly  a  bed  of  roses  for  the  engineer,  and  when  it  comes  to 
a  question  of  transmission  at  50,000  volts  or  so,  difficulties 
multiply.  At  and  above  this  pressure,  there  can  be  little 
doubt  that  insulation  is  a  very  difficult  task,  and  there  is 
equally  little  doubt  that  of  two  lines,  one  constant  current 
and  the  other  polyphase,  transmitting  the  same  energy  at  the 
same  effective  voltage,  the  former  would  be  in  trouble  much 
less  frequently  than  the  latter.  In  the  first  case  there  are  but 
two  wires  involved,  while  in  the  second  there  are  certain  to  be 
three,  and  generally  considerations  of  inductance  would  lead 
to  not  less  than  six  hi  a  plant  of  large  size.  And  when  the 
point  is  reached  where  insulation  is  a  costly  matter,  the  extra 
wires  and  precautions  may  easily  outweigh  any  intrinsic  saving 
in  copper.  The  constant  current  plant,  too,  always  has  the 
advantage  that  it  is  only  working  at  its  maximum  voltage 
during  the  peak  of  the  load  and  the  rest  of  the  time  has  a 
very  considerable  factor  of  safety. 

Whether  the  increased  cost  and  complication  of  the  gener- 
ating station  of  a  constant  current  system  can  ever  be  endured 
for  the  sake  of  these  advantages  is  a  matter  open  to  discussion ; 
it  certainly  cannot  be  answered  in  the  negative  offhand,  how- 
ever. The  continuity  of  service  possible  in  an  alternating 
plant  materially  above  50,000  volts  is  an  imknown  quantity, 
and  in  the  absence  of  data  upon  this  pohit  one  is  not  justified 
in  estimating  the  importance  of  an  alternative  method. 

It  is  a  mistake,  however,  to  suppose  that  the  considerably 


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TRANSMISSION  BY  CONTINUOUS  CURRENTS.        123 

increased  maximum  voltage  in  an  alternating  current  involves 
much  greater  danger  of  leakage,  or  of  breaking  down  insula- 
tion under  all  circumstances.  Under  many  conditions  it  is 
highly  probable  that  the  electrolytic  strain  from  continuous 
current  on  insulating  materials,  particularly  when  damp,  is 
more  destructive  than  the  added  electrostatic  strain  of  an 
alternating  current.  Within  any  voltages  now  regularly 
employed,  the  total  difference  is  probably  immaterial.  In  the 
matter  of  one  of  the  great  dangers  to  an  overhead  line  and 
apparatus,  i.e.,  injury  from  lightning,  direct  current  has  a  very 
material  advantage  in  that  it  is  possible  to  use  coils  of  con- 
siderable self-induction  in  connection  with  such  circuits,  so 
as  to  keep  oscillatory  discharges,  Uke  lightning,  out  of  the 
machines.  This  is  well  shown  in  the  singular  freedom  of  arc 
lighting  stations  from  serious  damages  to  the  machines  by 
lightning,  as  compared  with  stations  containing  other  kinds  of 
electrical  apparatus.  In  this  case  the  magnets  of  the  arc 
machines  themselves  act  as  a  powerful  ijiductance,  tending  to 
throw  the  lightning  to  earth.  High  voltage  shunt-wound 
dynamos  and  alternators  are  much  more  sensitive  in  this 
respect. 

Consequently,  part  of  the  price  one  has  to  pay  for  the  privi- 
lege of  utilizing  alternating  currents  is  extra  care  with  respect 
to  protective  devices  against  lightning.  In  the  present  state 
of  the  art,  the  best  field  for  combined  transmission  and  dis- 
tribution of  power  by  continuous  currents  is  in  cases  involving 
distribution  over  moderate  distances,  within,  say,  a  couple  of 
miles  from  the  centre  of  distribution,  and  even  then  in  prob- 
lems where  lighting  is  not  an  essential  part  of  the  work.  The 
voltage  of  a  lighting  circuit  is  determined  by  the  voltage  of 
the  lamps  which  can  be  employed  upon  it,  and  this  is  so 
limited  that  if  lighting  is  to  be  done  on  the  same  circuit  as 
power  distribution  there  are  few  cases  where  such  a  com- 
bhied  system  can  be  successfully  used  with  continuous  cur- 
rents. The  field  seems  at  present  to  be  somewhat  widened 
by  the  advent  of  3-wire  systems  at  220  to  250  volts  on  a  side, 
but  their  place  in  the  art  is  hardly  yet  secure,  although  their 
use  is  extending. 

At  all  long  distances  continuous  current  is  at  a  disadvantage 


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124  ELECTRIC  TRANSMISSION  OF  POWER. 

in  point  of  available  voltage  where  distribution  is  to  be  done, 
and  has  in  most  cases  no  very  material  advantages  for  single, 
unit  work.  It  will  require  considerable  further  advance  in 
dynamo  building  to  render  continuous  current  thoroughly 
available  for  high  voltages,  and  even  then  only  in  units  of 
moderate  size,  say  300  to  400  KW.  In  this  lies  its  weakness. 
Its  strength  is  largely  in  its  present  firm  foothold  in  electrical 
practice,  and  in  the  fact  that  standard  apparatus  for  continu- 
ous currents  is  available  everywhere,  and  is  manufactured 
cheaply  in  large  quantities  by  numerous  makers.  It  is,  fur- 
thermore, interchangeable  to  a  degree  which  will  never  be  true 
of  alternating-current  machinery  until  there  is  far  greater 
miity  in  alternating-current  practice  than  we  are  likely  to  have 
for  some  years  to  come. 


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CHAPTER  IV. 

SOME   PROPERTIES   OF   ALTERNATING   CIRCUITS. 

We  have  already  seen  in  Chapter  I  that  the  current  nor- 
mally produced  by  a  dynamo-electric  machine  is  an  alternating 
one,  SO  that  a  continuous  current  exists  in  the  external  circuit 
only  in  virtue  of  the  commutator.  Until  within  the  last 
fifteen  years  the  original  alternating  current  was  utilized  to 
but  a  trivial  extent.  Nevertheless,  it  possesses  certain  prop- 
erties so  valuable  that  their  practical  development  has  wrought 
a  revolution  in  applied  electricity. 

To  describe  these  properties  with  any  degree  of  complete- 
ness would  require  several  volumes  the  size  of  the  present,  and 
would  involve  mathematical  considerations  so  abstruse  as  to  be 
absolutely  imintelligible  to  any  save  the  professional  reader. 
We  shall  therefore  at  the  very  start  drop  the  academic  methods 
of  treatment  and  confine  ourselves,  so  far  as  possible,  to  the 
physical  facts  concerning  those  properties  of  alternating  cur- 
rents which  have  a  direct  bearing  on  the  electrical  transmission 
of  energy.  This  discussion  will  therefore  be  somewhat  imcon- 
ventional  in  form,  although  adhering  rigidly  to  the  results  of 
experiment  and  mathematical  theory.  The  student  who  is 
interested  in  the  exact  development  of  this  theory  will  do 
well  to  consult  the  excellent  treatises  of  Fleming,  Mascart 
and  Joubert,  Bedell  and  Crehore,  and  Steinmetz,  all  of  which 
are  full  of  valuable  demonstrations. 

The  fundamental  differences  between  the  behavior  of  con- 
tinuous and  of  alternating  currents  lie  in  the  fact  that  in  the 
former  case  we  deal  mainly  with  the  phenomena  of  a  flow  of 
electrical  energy  already  steadily  established,  while  in  the 
latter  case  the  phenomena  of  starting  and  stopping  this  flow 
are  of  primary  importance.  These  differences  are  akin  to  those 
which  exist  between  keeping  a  railway  train  in  steady  motion 
over  a  uniform  track,  and  bringing  it  up  to  speed  from  a 
state  of  rest.     In  steady  running  the  amount  of,  and  the 

125 


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126 


ELECTRIC  TRANSMISSION  OF  POWER. 


variations  in  the  power  needed,  depend  almost  wholly  on  the 
friction  of  the  various  parts,  while  in  starting  both  the  power 
and  its  variations  are  profoundly  affected  by  the  inertia  of  the 
mass,  the  elasticity  of  the  parts,  and  other  things  that  cut 
little  figure  when  the  train  is  up  to  a  uniform  speed. 

The  characteristic  properties  of  alternating  currents  are  due 
mainly  to  the  starting  and  stopping  conditions,  and  are  only  in- 
cidentally affected  by  the  circumstance  that  the  flow  of  current 
alternates  in  direction.  As,  however,  this  alternating  type  of 
current  is  in  general  use  and  its  uniform  oscillations  give  the 
best  possible  opportunity  for  observing  the  effect  of  repeated 
stops  and  starts,  we  will  look  into  the  generation  of  alternat- 
ing current,  not  forgetting  that  for  certain  purposes  we  shall 


FlO.  43. 

have  to  recur  to  the  phenomena  of  a  single  stop  or  start  in  the 
current. 

Fig.  43  shows  an  idealized  generator  of  alternating  currents. 
It  is  composed  of  a  single  loop  of  wire  arranged  to  turn  con- 
tinuously in  the  space  between  the  poles  of  a  magnet.  This 
space  is  a  region  of  intense  electromagnetic  stress  directed 
from  pole  to  pole,  as  indicated  by  the  dotted  lines.  The  two 
ends  of  the  loop  are  connected  to  two  insulated  metallic  rings 
connected  by  brushes  to  the  terminals  A  and  B  of  the  external 
circuit.  We  have  already  seen  that  what  we  call  electromotive 
force  appears  whenever  the  electromagnetic  stress  about  a 
conductor  changes  in  magnitude.  Now,  in  turning  the  loop  as 
shown  by  the  arrow,  the  electromagnetic  stress  through  it 
changes,  and  of  course  sets  up  an  electromotive  force.  In 
the  initial  position  of  the  loop  shown  in  Fig.  43,  it  includes 
evidently  the  maximum  area  under  stress;  after  it  has  turned 
through  an  angle  a,  this  area  will  be  much  lessened,  and  when 


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PROPERTIES  OF  ALTERNATING  CIRCUITS.         127 

a  =  90°,  the  loop  will  be  parallel  to  the  plane  of  the  electro- 
magnetic stress,  and  hence  can  include  none  of  it  at  all. 

But  the  resulting  electromotive  stress  is,  other  things  being 
equal,  proportional  to  the  rate  at  which  work  is  expended  in 
imiformly  turning  the  coil,  i.e.,  it  is  proportional  to  the  rate^ 
of  change  in  the  electromagnetic  stress  included  by  the  coil. 
This  rate  is,  during  a  single  revolution,  greatest  when  the 
sides  of  the  loop  are  moving  directly  across  the  lines  of  stress, 
and  least  when  moving  nearly  parallel  to  them.  Hence,  we  see 
from  Fig.  43  that  the  electromotive  force  in  our  coil  will  be  a 
maximum  when  a  =  90°  or  270°  and  a  minimum  in  the  two 
intermediate  positions.  For  a  simple  loop  it  is  easy  to  compute 
exactly  the  way  in  which  the  electromotive  force  will  vary  as 
the  loop  turns.     The  area  of  strain  included  by  the  coil  in  any 


FlO.  44. 

position  is  proportional  to  the  cosine  of  the  angle  a,  hence,  for 
uniform  motion  the  rate  of  change  in  the  area  is  proportional 
to  the  sine  of  o.  Therefore,  the  E.  M.  F.  at  every  point  of  the 
revolution  is  proportional  to  sine  a. 

If  now  we  draw  a  horizontal  line  and  measure  along  it  equal 
distances  corresponding  to  degrees,  and  then,  erecting  at  each 
degree  a  line  in  length  proportional  to  the  sine  of  that  par- 
ticular angle,  join -the  ends  of  these  perpendiculars,  we  shall 
have  an  exact  picture  of  the  way  in  which  the  E.  M.  F.  of 
our  loop  rises  and  falls.  Fig.  44  is  such  a  curve  of  E.  M.  F.  — 
a  so-called  "sine  wave,"  which  is  expressed  by  the  equation, 
E  =  E^  sin  at. 

This  simple  form  of  E.  M.  F.  curve  —  the  "sine  wave"  —  is 
assumed  to  exist  in  most  mathematical  discussions  of  alternat- 
ing current  to  avoid  the  frightful  complications  which  would 


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128 


ELECTRIC  TRANSMISSION  OF  POWER. 


result  from  assuming  such  E.  M.  F.  curves  as  often  are  foimd  in 
practice.  This  assumption  is  somewhat  rash,  for  a  true  sine 
wave  is  never  given  by  any  practical  generator,  but  the  error 
does  not  often  invalidate  any  of  the  conclusions,  for  the  exact 
form  of  the  wave  only  matters  in  discussing  certain  cases,  where 
it  can  often  be  taken  into  account  without  much  difficulty. 

Actual  alternating  generators  give  curves  of  E.  M.  F.  greatly 
influenced  by  the  existence  of  an  iron  armature  core  which 
collects  the  lines  of  force  so  that  as  the  core  turns  the  change 
of  stress  through  the  armature  coils  is  not  directly  propor- 
tional to  anything  in  particular.  A  glance  at  the  rudimentary 
dynamo  of  Fig.  8,  Chapter  I,  will  suggest  the  reason.     It  is  evi- 


Fig.  46. 


dent  enough  that  the  armature  could  turn  almost  30°  from  the 
horizontal  with  scarcely  any  change  in  the  magnetic  relations 
of  the  coils.  The  result  would  be  a  wave  with  a  very  flat  and 
depressed  top,  since  the  rate  of  change  of  induction  would  be 
very  moderate  when  it  should  be  considerable.  The  practical 
bearings  of  wave  form  on  power  transmission  work  will  be  taken 
up  in  the  next  chapter.  At  present  it  will  suffice  to  say  that  the 
best  standard  alternators  give  a  fairly  close  approximation  to 
the  sine  form.  Fig.  45  shows  the  E.  M.  F.  curve  of  one  of  the 
great  Niagara  generators.  This  is  an  excellent  example  of 
modem  practice  and  shows  a  form  slightly  flatter  than  the  sine 
curve  and  with  a  mere  trace  of  depression  at  the  crest.  Plenty 
of  machines  are  in  operation,  however,  that  give  curves  not 
within  hailing  distance  of  being  sinusoidal  —  e.g.  Fig.  46,  which 
shows  the  E.  M.  F.  curve  of  one  of  the  earliest  alternators 


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PROPERTIES  OF  ALTERNATING  CIRCUITS,  129 

designed  for  electric  welding.  In  this  case  there  is  a  far  sharper 
wave  than  the  sinusoidal,  of  a  curious  toothed  form.  Many  of 
the  early  alternators  with  ironclad  armatures  gave  curves  quite 
far  from  the  sine  form,  generally  rather  pointed,  while  the 
tendency  in  recent  machines  has  been  rather  in  the  opposite 
direction,  toward  curves  like  Fig.  45,  although  seldom  so  nearly 
sinusoidal.  The  general  equation  for  their  E.  M.  F.  is, 
£=J?iSina/  +  ^8sin3a/+^5sin5a^  .   .   .  E(2n-i)sin(2n—l)at. 


FlO.  46. 

In  other  words,  the  E.  M.  F.  is  built  up  of  the  fundamental 
frequency  and  its  odd  harmonics. 

Now  as  to  the  current  produced  by  this  oscillating  electro- 
motive force.  In  ordinary  work  with  continuous  currents,  the 
current  corresponding  to  each  successive  value  of  the  E.  M.  F. 
would  be  very  easy  to  determine  by  simple  reference  to  Ohm's 

E 
law,  C  =  -  •    If  the  dynamo  of  Fig.  43  gave  one  volt  maximum 
R 

E.  M.  F.  and  were  connected  through  a  simple  circuit  of  one 

ohm  resistance  the  maximum  current  would  be  one  ampere, 

and  the  current  at  all  points  would  be  directly  proportional  to 


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130  ELECTRIC  TRANSMISSION  OF  POWER. 

the  voltage.  Hence  if  the  E.  M.  F.  varied  as  shoT\7i  in  Fig.  44, 
the  current  would  vary  in  precisely  the  same  manner,  and  the 
curve  showing  its  variation  would,  if  drawn  to  the  same  scale, 
exactly  coincide  with  the  curve  of  Fig.  44.  This  would  be 
generally  true  if  we  had  only  resistance  to  consider,  and  the 
treatment  of  alternating  currents  would  then  be  very  simple. 

But  the  starting  and  stopping  of  current  which  takes  place 
periodically  in  alternating  circuits  produces  great  changes  in 
the  electro-magnetic  stresses  about  the  conductors,  and  these 
changes  are  in  turn  capable  of  very  important  reactions. 
They  give  to  the  alternating  current  its  most  valuable  proper- 
ties, but  also  involve  its  action  in  very  curious  complications. 

Turn  back  to  Chapter  I  and  examine  Fig.  4.  We  see  from 
it  that  whenever  the  electro-magnetic  stresses  about  a  circuit 


mm 


«  / 


Fig.  47. 

as  A,  change  by  the  variation  of  the  current  flowing  in  it,  an 
E.  M.  F.  is  set  up  in  the  parallel  circuit  B^  opposing  the 
change  of  E.  M.  F.  in  A.  This  fact,  as  we  shall  see  later,  is 
the  root  of  the  alternating-current  transformer.  Suppose  now 
that  in  the  circuit  of  our  alternator  is  a  coil  of  wire  woimd  in 
close  loops  as  shown  in  Fig.  47.  Here  A  and  B  are  the  dyna- 
mo terminals,  C  the  general  circuit,  and  D  the  aforesaid  coil. 
Let  an  E.  M.  F.  be  started  m  the  direction  A  C  D.  The  result- 
ing current  flows  through  D,  but  the  electro-magnetic  stresses 
set  up  by  the  current  about,  for  instance,  the  loop  e,  produce 
an  E.  M.  F.  in  neighboring  coils  tending  to  drive  current  in 
a  direction  opposite  to  that  from  the  dynamo.  In  other 
words,  e  acts  toward  /  just  as  A  acted  toward  B  in  Fig.  4. 
Thus  each  turn  tends  to  oppose  the  increase  of  current  in 
the  others.  When  the  current  in  C  ceases  to  vaiy,  of  course 
the  reactive  E.M.  F.'s  in  e  and  /  stop,  for  there  is  for  the  moment 
no  change  of  stress  to  produce  them,  but  as  the  main  current 
begins  to  decrease,  the  reactive  E.  M.  F.'s  set  in  again. 


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PROPERTIES  OF  ALTERNATING  CIRCUITS.  131 

The  main  or  "impressed"  E.  M.  F.  is  thus  opposed  in  all 
its  changes  by  the  reactive  or  "inductive"  E.  M.  F.  due  to 
the  combined  action  of  the  loops  at  D.  Hence  tlie  impressed 
E.  M.  F.  in  driving  current  through  the  system  has  to  over- 
come, not  only  the  resistance  of  the  conductors,  but  opposing 
electromotive  forces.  Therefore  since  a  part  of  the  impressed 
E.  M.  F.  is  taken  up  with  neutralizing  the  inductive  E.  M.  F., 
only  the  remainder  is  effective  against  the  true  resistance  of 
the  circuit.  Ohm's  law,  then,  cannot  apply  to  alternating  cir- 
cuits in  which  there  is  inductive  action,  except  in  so  far  as  we 
deal  with  the  "effective"  E.  M.  F.    The  relation  between  the 

E  E 

impressed  E.  M.  F.  and  the  current  is  not  C  =  —f  but  C  =  — 

R  R 


FlO.  48. 

less  a  quantity  depending  on  the  amoimt  of  inductive  E.  M.  F. 
encountered. 

This  state  of  things  leads  to  two  very  important  results: 
First,  the  current  in  an  inductive  circuit  is  less  than  the  im- 
pressed E.  M.  F.  would  indicate.  Second,  this  current  reaches 
its  maximum  later  than  the  impressed  E.  M.  F.  For  the 
current  depends  on  the  effective  P].  M.  F.,  and  for  each  partic- 
ular value  of  this  the  impressed  E.  M.  F.  must  have  had  time 
to  rise  enough  to  overcome  the  corresponding  value  of  the 
inductive  E.  M.  F.  The  current  is  thus  damped  in  amount 
and  caused  to  lag  in  "phase"  as  sho\^Ti  in  Fig.  48.  The  heavy 
line  here  shows  the  variations  of  the  impressed  E.  M.  F.,  and 
the  light  line  the  corresponding  variations  of  current  in  a 
circuit  containing  inductive  reaction  —  inductance.  The  dis- 
tance a  b  represents  the  "angle  of  lag,"  while  6  c  is  180°  as 
shown  in  Fig.  44.  Very  similar  relations  are  found  in  prac- 
tice, although  the  lag  is  often  greater  than  shown  in  the  cut, 


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132  ELECTRIC  TRANSMISSION  OF  POWER. 

particularly  when  alternating  motors  are  in  circuit.  Fig. 
49  shows  the  curves  of  E.  M.  F.  and  current  from  a  very  small 
alternating  motor  at  the  moment  of  starting.  The  angle  of 
lag  in  this  case  is  a  trifle  over  45*^,  and  the  curves  are  much 
closer  to  sine  waves  than  is  usual. 

The  inductive  E.  M.  F.,  as  has  already  been  explained,  is  due 
to  the  magnetic  changes  produced  by  the  variation  of  the  cur- 
rent. Just  as  in  the  d3aiamo  of  Fig.  43,  the  actual  amount  of 
E.  M.  F.  is  directly  proportional  to  the  rate  of  change  in  mag- 
netic stress,  which  is  in  turn  proportional  to  the  change  of  cur- 
rent. The  inductive  E.  M.  F.  is  therefore  at  every  point 
proportional  to  the  rate  of  variation  of  the  current.     But  the 


PlO.  49. 

current  wave  is,  like  the  impressed  E.  M.  F.  wave,  still  approxi- 
mately a  sine  curve,  for  it  has  been  merely  shifted  back 
through  the  angle  of  lag,  and  although  damped,  it  has  been 
simply  changed  to  a  different  scale.  Being  still  essentially 
a  sine  curve,  its  rate  of  variation  is  a  cosine  curve,  or,  what 
is  the  same  thing,  a  sine  curve  shifted  backward  a  quarter 
period,  90®.  Indeed,  this  is  at  once  evident,  for,  since  the 
current  varies  most  slowly  at  its  maximum,  the  inductive 
E.  M.  F.  must  be  a  minimum  at  that  point,  i.e.,  it  must  be 
90®  behind  the  current  in  phase,  while  since  E.  M.  F.  and  cur- 
rent vary  symmetrically,  in  general  the  forms  of  the  two  curves 
will  be  similar.  The  effective  E.  M.  F.  which  is  actually 
engaged  in  driving  the  current  is  a  wave  in  phase  with  the 
current  it  drives,  and  of  similar  shape,  i.e.,  a  sine  curve. 


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PROPERTIES  OF  ALTERNATING  CIRCUITS. 


183 


We  have,  then,  in  an  inductive  circuit  three  E.  M.  F.'s  to  be 
considered : 

I.   The  impressed  P].  M.  F.,  acting  on  the  circuit. 
II.   The  inductive  E.  M.  F.,  opposing  I. 

III.   The  effective  E.  M.  F.,  the  resultant  of  I  and  II. 

Plotting  the  respective  curves,  they  bear  to  each  other  the 
relation  shown  in  Fig.  50.  Here  a  is  the  impressed  E.  M.  F., 
b  the  effective  E  M.  F.  (or  the  current)  lagguig  behind  a 
through  an  angle  usually  denoted  by  <A,  and  c  is  the  induc- 
tive E.  M.  F.  90°  behind  b.  Now  since  b  is  the  resultant  of 
the  interaction  of  a  and  c,  and  we  know  that  b  and  c  are  90° 
apart  in  phase,  it  is  comparatively  easy  to  find  the  exact  rela- 
tion between  the  three. 

For  we  can  treat  electromotive  forces  acting  at  known  angles 
with  each  other  just  as  we  would  treat  any  other  forces  work- 


Fio.  50. 


Fig.  61. 


ing  conjointly.  If  for  example  we  have  a  force  A  B,  Fig.  51, 
acting  simultaneously  with  a  force  B  C,  at  right  angles  to  it, 
the  magnitudes  of  the  forces  being  proportional  to  the  lengths 
of  the  lines,  the  result  is  the  same  as  if  a  single  force  in  magni- 
tude and  direction  A  C  were  working  instead  of  the  two  com- 
ponents. This  is  a  familiar  general  theorem  that  proves  par- 
ticularly useful  in  the  case  in  hand. 

If  we  take  A  B  equal  to  the  effective  E.  M.  F.,  and  B  C  equal 
to  the  inductive  E.  M.  F.,  then  ^1  C  is  the  impressed  E.  M.  F. 
It  at  once  appears  that  the  angle  between  A  B  and  A  C  is  <l>j 
the  angle  of  lag.  Then  from  elementarj*^  trigonometry  it 
appears  that 

AB  ^  ACcos<l> 

CB  =  ACsm<l>  =  AB  tan  <^,  hence 

CB 
tan<A=.— . 


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134  ELECTRIC   TRANSMISSION  OF  POWER, 

We  are  therefore  in  a  position  to  determine  the  three  E.  M. 
F.'s  and  the  angle  <^,  knowing  any  two  of  the  four  quantities. 
Thus  for  a  given  impressed  E.  M.  F.,  £,  such  as  is  found  on 
any  constant-potential  alternating  circuit,  the  effective  E.  M. 
F.  which  determines  the  current  is  given  by  E  cos  <l>.  As  <l> 
grows  less  and  less  through  decrease  of  the  inductive  E.  M.  F., 
A  Cf  the  impressed  E.  M.  F.  necessary  for  a  given  current 
also  decreases,  and  finally,  when  <l>  becomes  zero,  A  C  =  A  B. 
In  other  words,  the  impressed  E.  M.  F.  is  then  simply  that 
needed  to  overcome  the  ohmic  resistance. 

For  any  particular  current,  then,  A  B  is  directly  propor- 
tional to  the  resistance  of  the  circuit,  while  C  B  is  directly 
proportional  to  the  "inductance"  of  the  circuit,  that  prop- 
erty of  the  particular  circuit  which  determines  the  inductive 
E.  M.  F.  Calling  this  /  we  may  redraw  Fig.  51  in  a  very 
convenient  form  —  Fig.  52.  Here  we  see  the  relation  between 
R  and  /  in  determining  the  impressed  E.  M.  F.  necessary  to 
drive  a  certain  current  through  an  inductive  circuit.  The 
magnitude  of  the  E.  M.  F.  evidently  is  Vfl^  +  /'  if  the  units 
of  measurement  are  chosen  correctly,  and  it  is  always  pro- 
portional to  this  quantity,  which  is  related  to  the  impressed 
E.  M.  F.  as  resistance  is  to  the  effective  E.  M.  F. 

Hence  v  i2'  +  P  has  sometimes  been  called  "apparent 
resistance."  The  more  general  name,  however,  is  impedance, 
which  indicates  the  perfectly  general  relation  between  E.  M.  F. 
and  current.  If  /  be  zero,  as  in  a  continuous-current  circuit, 
then  the  impedance  becomes  the  simple  resistance.  We  can 
now  write  out  some  of  the  general  relations  of  current  and 
E.  M.  F.  in  alternating  circuits  as  follows,  calling  E  the  im- 
pressed E.  M.  F.  as  before: 

E 
C  =    ^  K"  ■\-  P 
E  ^  C  '^  K"  +  P 

and  with  respect  to  the  angle  of  lag, 
I ^ 

y R?  +  p-  ^ 


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PROPERTIES  OF  ALTERNATING  CIRCUITS. 


135 


/  =  ie  tan  <^ 

tan  <^  =  -  . 
K 

Hence,  knowing  the  angle  of  lag  and  the  resistance  of  a 
circuit,  the  inductance  can  be  found  at  once.  The  angle 
of  lag,  depending  on  the  ratio  of  /  and  iJ,  must  be  the  same 
for  all  circuits  in  which  this  ratio  is  the  same.  Also  in 
any  circuit  of  given  inductance,  increasing  the  resistance 
diminishes  the  angle  of  lag,  while  of  course  also  diminishing 
the  current  for  a  given  value  of  E,  In  fact,  since  /  does  not 
represent  work  done,  for  the  inductive  E.  M.  F.  represents 


FlO.  S3. 


merely  a  certain  amount  subtracted  from  the  impressed 
E.  M.  F.  by  the  reaction  of  the  circuit,  any  process  which  for 
a  given  value  of  E  increases  the  energy  actually  spent  in  the 
circuit  is  accompanied  by  a  diminution  of  the  angle  of  lag. 

This  freedom  of  the  circuit  from  any  energy  losses  due  to 
/  is  a  fact  of  the  greatest  importance.  It  is  fully  borne  out 
by  experiment,  and  there  is  besides  good  physical  reason  for 
it.  For  since  current  and  E.  M.  F.  are  the  two  factors  of 
electrical  energy,  there  can  be  no  energy  when  the  product  of 
these  factors  is  zero.  Note  now  Fig.  53,  developed  from  Fig. 
'50.  Hence  a  is  the  line  of  zero  E.  M.  F.  and  current,  h  the 
current  curve  for  a  single  alternation,  and  c  the  correspond- 
ing curve  of  inductive  E.  M.  F.  90°  behind  the  current. 
When  6  is  a  maximum,  c  is  zero,  and  vice  versa.  And  since 
c  is  equally  above  and  below  the  zero  line  during  each  alterna- 
tion of  current,  the  average  E.  M.  F.  is  zero,  and  therefore 
the  average  energy  throughout  the  alternation  is  zero.    The 


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136  ELECTRIC   TRANSMISSION  OF  POWER, 

same  conditions  would  evidently  continue  if  instead  of  an 
alternation  we  took  a  complete  cycle  (i.e.,  the  whole  curve 
from  the  time  the  current  starts  in  a  given  direction  until 
it  starts  in  the  same  direction  again)  or  any  number  of  cycles. 
Thus  /  must  be  entirely  dropped  out  of  consideration  in  dis- 
cussing the  question  of  work  done  in  an  alternating  circuit. 
And  since  E  differs  from  £4,  the  effective  E.  M.  F.,  only  by 
a  fimction  of  /,  the  energy  value  of  which  is  zero,  the  energy 
in  the  circuit  is  exactly  measured  by  E^  and  the  corresponding 
current  which,  as  we  have  seen,  is  in  phase  with  it.     But 

El  =  E  cos  <l>. 

Hence,  multiplying  both  members  of  this  equation  by  C  to 
reduce  to  energy. 

Energy  =  C  ^j  =  C  £  cos  <^. 

That  is,  the  energy  in  an  alternating  circuit  is  equal,  not  to 
the  impressed  E.  M.  F.  multiplied  by  the  current,  but  to  their 
product  multiplied  by  the  cosine  of  the  angle  of  lag.  The 
product  C  E  is  sometimes  called  the  apparent  energy  to  dis- 
tinguish it  from  C  E^y  the  actual  energy.  This  apparent 
energy  is  that  obtained  by  measuring  the  amperes  and  the 
impressed  volts  and  taking  their  product.  The  real  energy 
is  that  which  would  be  obtained  by  putting  a  wattmeter  in 
circuit.     Hence 

watts 
volt-amperes 

a  convenient  and  common  method  of  measuring  the  angle  of 
lag.  If  in  addition  the  value  of  /  is  wanted,  it  can  be  obtained 
at  once  from  the  expression  for  tangent  <^  already  given. 

We  thus  see  that  the  energy  in  an  inductive  circuit  is  not 
directly  proportional  to  the  voltage  as  measured,  but  to  the 
effective  voltage,  which  is  less  by  an  amount  depending  on 
the  inductance.  This  difference  is  sometimes  referred  to  as 
the  ''inductive  drop"  in  a  circuit.  The  result  is  that  to  drive 
a  given  current  through  an  inductive  circuit  the  generator 
nuist  give  a  voltage  depending  on  the  impedance  of  the  cir- 
cuit.    On  the  other  hand,  if  an  inductive  circuit  be  fed  from 


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PROPERTIES  OF  ALTERNATING  CIRCUITS.  137 

a  given  impressed  E.  M.  F.  the  current  required  to  represent 
a  given  amount  of  energy  exceeds  that  required  in  a  non-induc- 
tive circuit,  in  the  ratio  of  1  to  the  cosine  of  the  angle  of  lag. 
The  net  result,  then,  of  inductance  in  an  alternating  circuit  is 
to  increase  the  E.  M.  F.  at  the  generator  required  to  produce 
a  given  E.  M.  F.  at  the  load,  and  to  increase  the  current  re- 
quired to  deliver  a  given  amoimt  of  energy. 

The  E.  M.  F.  and  current  are  here  supposed  to  be  meas- 
ured in  the  ordinary  way,  by  properly  designed  voltmeters 
and  ammeters.  In  power  transmission  work,  inductance  in  the 
circuit  (line  or  load  or  both)  means  that  the  dynamo  has  to 
give  voltage  enough  to  overcome  the  impedance  of  the  system 
and  still  to  deliver  the  proper  number  of  volts  at  the  motor, 
while  the  motor  will  take  extra  current  enough  to  compensate 
for  the  lag  between  the  E.  M.  F.  at  its  terminals  and  the  re- 
sulting current. 

The  dynamo  thus  has  to  be  capable  of  giving  a  little  extra 
voltage,  and  the  motor  must  be  able  to  stand  a  little  extra  cur- 
rent. In  other  words,  both  machines  must  have  sufficient  mar- 
gin in  capacity  to  take  care  of  this  matter  of  lagging  current. 

We  have  already  seen  the  general  relation  between  resist- 
ance, inductance,  and  impedance.  Let  us  now  look  into  the 
quantity  last  mentioned  so  as  to  see  its  numerical  relation  to 
the  others.  If  a  circuit  has  a  certain  resistance  in  ohms  and 
a  given  inductance,  what  is  its  impedance,  i.e.,  the  ratio 
between  the  measured  voltage  and  the  measured  current? 

The  real  question  involved  is  the  value  of  the  inductive  E. 
M.  F.  This,  like  any  other  E.  M.  F.,  is  proportional  to  the 
rate  of  variation  of  the  electro-magnetic  stress  which  produces 
it.  Its  total  magnitude  depends  on  the  rate  of  variation  of  the 
current  and  the  ability  of  this  current  to  set  up  stresses  which 
can  affect  neighboring  conductors  as  in  Fig.  43.  This  latter 
property  depends  on  the  number  of  turns,  their  locality  with 
reference  to  each  other,  and  other  similar  conditions  which 
depend  simply  on  the  physical  nature  of  the  circuit,  and  so  for 
any  given  circuit  are  settled  once  for  all.  These  properties 
are  defined  on  the  basis  of  their  net  effect,  and  the  ratio  of  the 
rate  of  variation  of  the  current  to  the  inductive  E.  M.  F.  pro- 
duced by  it  in  a  given  circuit  is  usually  known  as  L,  the 


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188  ELECTRIC  TRANSMISSION  OF  POWER. 

"coefficient  of  self-induction"  of  that  circuit.  The  total 
inductive  E.  M.  F.  is  then  equal  to  L,  multiplied  by  the  actual 
rate  of  current  variation  expressed  in  such  units  as  will  fit  the 
general  system  by  which  E,  R  and  other  quantities  are  con- 
cord an  tly  measured. 

Expressed  in  this  way,  the  rate  of  current  variation  in  an 
alternating  circuit  is  2  «■  n,  where  n  is  the  number  of  cycles  per 
second,  and  v  has  its  ordinary  meaning  of  3.1416.  Hence  the 
inductance  of  the  circuit  is  numerically  2  v  n  L,  the  last  factor 
being  dependent  on  the  nature  of  the  circuit  and  denoting 
the  inductance  per  unit  rate  of  current  variation.  The  2  tt  n 
factor  gives  the  actual  rate  of  current  variation,  which  may 
change  to  any  amount,  while  L  remains  fixed.  L  therefore 
may  at  all  times  in  a  given  circuit  be  expressed  in  terms  of 


any  unit  that  is  conveniently  related  to  other  electrical  units. 

Such  a  unit  inductance  is  the  henry,  which  is  the  inductance 
corresponding  to  an  inductive  E.  M.  F.  of  1  volt  when  the 
inducing  current  varies  at  the  rate  of  1  ampere  per  second. 

If,  therefore,  L  for  any  circuit  is  known  in  henrys,  the 
total  inductance  /  is  6.28  n  L. 

We  are  now  ready  to  apply  a  numerical  value  to  /  in  Fig. 
52  and  the  resulting  equations. 

For  example,  let  us  suppose  that  a  certain  alternating  circuit 
has  a  resistance  of  100  ohms  and  L  =  0.1  henry.  The  im- 
pressed E.  M.  F.  is  1,000  volts.  What  will  be  the  current  and 
its  angle  of  lag?  Lay  off  A  B,  Fig.  54, 100  units  long.  Then 
at  B,  to  the  same  scale  erect  a  perpendicular  B  C,  2  ir  n  L  in 
height.  If  we  are  dealing  with  an  alternating  circuit  of  60  ^ 
per  second,  such  as  is  often  used  for  power  transmission,  2 
IT  n  L  will  be  37.7  units  high.  Now  join  A  C,  and  the  result- 
ing length  on  the  same  scale  is  the  impedance  in  ohms.     But 


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PROPERTIES  OF  ALTERNATING  CIRCUITS.  139 

A  C  =  Vi22  +  p  =  Vl00»  +  37.72  _  106.9  nearly.  And  since 
the  current  equals  the  impressed  E.  M.  F.  divided  by  the  im- 
pedance, the  current  in  tliis  case  would  be  9.36  amperes  instead 
of  the  10  amperes  due  if  there  had  been  no  inductance.     And 

since  tan  <^  =  ■- ,  it  is  here  .377,  which  corresponds  to  an  angle 

R . 

of  20°  40'. 

Also,  since  E  =-  C  y^+P,  we  can  readily  find  the  im- 
pressed E.  M.  F.  required  to  produce  in  this  circuit  any  given 
current.     For  C  =  10  amperes,  E  =  1,069  volts,  and  so  on. 

w^atts 

We  have  seen  that  cos  <^  =  — so    that    in    the 

volt-amperes 

case  in  hand  where  cos  <^  =  .936  the  actual  energy  in  the  cir- 
•cuit  is  93.6  per  cent  of  that  indicated  by  the  readings  of  volt- 
meter and  ammeter. 

This  factor,  cos  <^,  connecting  the  apparent  and  the  real 
energy,  is  known  as  the  "power  factor"  of  the  circuit. 

As  I  —  R  tan  <^,  and  in  any  given  case  n  is  known,  L  can 
readily  be  obtained  from  a  measurement  of  lag  in  a  circuit  of 
known  resistance.  It  must  be  remembered,  however,  that  if 
the  inductance  is  due  to  a  coil  having  an  iron  core,  the  value 
of  L  will  change  when  the  magnetization  of  the  iron  changes, 
so  that  results  obtained  with  a  certain  current  will  not  hold 
exactly  for  other  currents.  The  values  of  L  found  in  practice 
cover  a  very  wide  range,  from  a  few  thousandths  of  a  henry  in 
a  small  bit  of  apparatus  like  an  electric  bell,  to  some  hundreds 
of  henrys  in  the  field  magnets  of  a  big  dynamo.  L  in  fact  is 
nearly  as  variable  as  R. 

As  a  practical  example  in  inductance  effects  we  may  consider 
the  effect  of  alternating  current  in  a  long  straightaway  circuit. 
Suppose  for  example  we  have  a  circuit  50,000  ft.  long  composed 
of  No.  4  B.  &  S.  copper  wires.  The  wires  are  about  1  ft.  apart 
and  about  20  ft.  above  the  ground.  What  voltage  will  be 
required  to  deliver  10  amperes  through  this  circuit  at  130 
cycles  per  second,  and  what  will  be  the  angle  of  lag?  The 
resistance  of  this  wire  is  0.25  ohms  per  1,000  ft.  L,  its  co- 
efficient of  induction,  is  .0003  henry  per  1,000  ft.  The  total 
resistance  of  the  circuit  is  then  25  ohms,  and  its  total  induc- 


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140 


ELECTRIC   TRANSMISSION  OF  POWER, 


tance,  /,  =  6.28  x  130  X.03  =  24.5.     Plotting  as  before  these 

values,  in  Fig.  55  we  have  the  impedance  equal  to    v25M-24^ 

=  35  ohms.     Hence  E  must  be  350  volts  instead  of  the  250 

that  would  suffice  in  the  case  of  continuous  current.       Tan 

24.5 

i>  = =  .98.     The  corresponding  angle  is  44°  25'.    The 

^o 

ratio  of  impedance  to  resistance  in  this  case  is  1.4:1.     This 

ratio,  often  called  the  impedance  factor,  is  a  very  convenient 

way  of  treating  the  matter,  and  tables  giving  its  value  for 

common  cases  will  be  given  later.     In  case  of  apparatus  being 

connected  to  the  circuit,  the  computation  of  its  effect  is  easy. 


If  it  has  resistance  R^  and  inductance  P  then  the  total  impe- 
dance of  the  circuit  will  be  V(/e  -f  R^y  +  (/  +  ^Y  and  so 
on  for  any  number  of  resistances  and  inductances,  the  impe- 
dance being  always  equal  to  the  square  root  of  the  squared 
sum  of  the  resistances  plus  the  squared  sutn  of  the  inductances. 
Thus  an  inductance  added  anywhere  in  circuit  changes  the 
total  impedance  and  the  angle  of  lag. 

There  are  several  ways  of  looking  at  inductance,  according 
as  one  wishes  to  deal  more  particularly  with  inductive  E.  M.  F., 
the  changes  in  electro-magnetic  stress  which  produce  it,  or 
the  energy  changes  which  accompany  it.  The  first  point  of 
view  is  the  one  here  taken,  in  accordance  with  the  definition 
of  the  henry  just  given.  Hence  the  henry  may  be  called  unit 
inductance,  in  which  case  the  quantity  /  which  we  have  been 
considering  measures  the  inductive  E.  M.  F.,  and  since  it  is 


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PROPERTIES  OF  ALTERNATING  CIRCUITS.  I4l 

the  product  of  the  inductance  for  unit  rate  of  current  change 
multiplied  by  2  w  n,  it  is  sometimes  referred  to  as  inductance- 
speed,  now  conventionally  termed  reactance. 

In  alternating-current  working  inductance  may  easily  be- 
come quite  troublesome,  through  the  '^inductive  drop"  in 
the  line  and  the  necessity  of  sometimes  delivering  a  current 
quite  out  of  proportion  to  the  energy.  Thus  in  alternating- 
current  lighting  plants  during  the  hours  of  daylight  when  the 
actual  load  is  small,  the  current  may  be  of  quite  imposing  size 
from  the  lag  produced  by  the  inductance  of  the  unloaded 
transformers  in  circuit.  The  sort  of  thing  which  happens  may 
readily  be  figured  out.  Suppose  we  are  dealing  with  a  trans- 
former or  other  inductive  apparatus  having  a  resistance  of  5 
ohms  and  L  =  1  henry.  The  impedance  at  60  <^  will  then  be 
V  5^  +  (6.28  X  60  X  ly  ==  V25  +  376.8^  =  377.8  ohms,  sub- 
stantially  the  same  as  the  inductance  alone,  and  under  an 
impressed  E.  M.  F.  of  1,000  volts  the  resulting  current  would 

377  8 
be  2.65  amperes.    But  tan  <^  =  — -—  =  75.56.   Hence  <l>  =  89°15' 

5 

and  cos  <l>  =  .013.  Therefore  while  the  apparent  energy  is 
2.65  X  1,000  =  2,650  watts,  the  real  energy  is  only  2,650  X.013 
watts  =  34  -I- :  really  the  loss  due  to  heating  the  conductor. 
This  is  of  course  a  very  exaggerated  case,  as  it  takes  no  account 
of  the  energy  that  would  be  required  to  reverse  the  magneti- 
zation in  whatever  iron  core  the  apparatus  might  have.  It 
does,  however,  show  very  clearly  that  the  current  flowing 
depends  practically  on  the  inductance  and  very  little  on  the 
resistance,  and  that  the  angle  of  lag  is  so  great  that  the  dis- 
crepancy between  apparent  and  real  energy  may  also  be  very 
great.  In  practice  cos  <^  may  fall  as  low  as  0.1  on  single 
pieces  of  apparatus,  and  ranges  up  under  varying  conditions 
of  load  to  .95  or  more. 

These  practical  considerations  naturally  raise  a  question  as 
to  the  effect  of  impedances  in  parallel.  The  joint  impedance 
of  two  impedances  in  series  must  first  be  discussed. 

The  resistance  of  two  resistances  in  parallel  is  of  course 
familiar.  If  i?  =  2  ohms  and  i2^  =  4  ohms,  then  their  joint 
resistance  ig  the  reciprocal  of  the  sum  of  their  reciprocals, 


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142 


ELECTRIC  TRANSMISSION  OF  POWER. 


thus, 


(R  +  R')  = 


i  +  i 


=  1 J  ohms. 


We  have  seen,  however,  that  impedances  cannot  be  added 
in  the  ordinary  manner.  If  we  take  two  impedances  made  up 
respectively  of  i?  =  4,  /  =  3,  and  R^  =  6,  /'  =  3,  we  must  pro- 
ceed as  in  Fig.  56.  The  first  impedance  is  5,  the  second  6.70. 
The  true  impedance  of  the  two  in  series  is  given  by  the  dotted 
Unes,  and  is  11.66,  not  11.70.  That  is,  the  impedances  must  be 
added  geometrically,  since  unless  <A  =  <^i  the  arithmetical  sum 
of  the  impedances  does  not  represent  the  facts  in  the  case. 


■r^7 


FlO.  66. 


Similarly,  while  it  is  perfectly  true  that  the  joint  impedance 
of  two  impedances  in  parallel  is  equal  to  the  reciprocal  of  the 
•sum  of  their  reciprocals,  the  summation  must  be  done  as  in 
Fig.  56  to  take  account  of  the  difference  of  phase  which  may 
exist  in  the  two  branches.  Taking  the  data  just  given,  the 
reciprocals  of  the  two  impedances  are  .20  and  .149  respectively. 
Drawing  these  on  any  convenient  scale  as  in  Fig.  57,  preserv- 
ing between  them  the  angle  due  to  the  difference  of  phase  as 
given  by  <l>  and  <^4,  we  find  the  geometrical  sum  of  the  recipro- 
cals to  be  .348,  of  which  the  reciprocal  is  2.87.  This  is  the 
joint  impedance  of  the  two  which  we  have  thus  geometrically 
added. 

This  same  process  can  be  extended  to  any  number  of  impe- 
dances in  parallel.  In  a  precisely  similar  way  any  number  of 
directed  quantities  may  be  laid  off  and  geometrically  added, 
the  final  sum  being  in  direction  and  magnitude  the  line  from 


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PROPERTIES  OF  ALTERNATING  CIRCUITS.  143 

the  starting  point  to  the  finish.  It  is  important  to  note  that 
since  the  currents  in  such  cases  are  generally  not  in  phase 
with  each  other,  it  usually  happens  that  the  sum  of  the  currents 
in  the  branches  differs  from  the  current  in  the  main  circuit, 
as  they  are  ordinarily  measured.  It  is  in  fact  a  prominent 
characteristic  of  alternating  circuits  that  both  currents  and 
voltages  are  liable  to  vary  in  a  way  at  first  sight  very  erratic. 
Particularly  is  this  the  case  when  there  is  capacity  in  the  cir- 
cuit, a  condition  which  we  will  now  investigate. 

By  a  circuit  having  capacity,  we  mean  one  so  constituted 
that  E.  M.  F.  applied  to  it  stores  up  energy  in  the  form  of 
electrostatic  stress,  which  starts  this  energy  back  in  the  form 
of  current  when  the  constraining  E.  M.  F.  is  removed. 

Such  a  condition  exists  whenever  two  conductors  are 
separated  by  an  insulating  medium,  or  dielectric,  as  in  the 
ordinary  condenser  of  Fig.  58.     Here  A  and  B  are  two  metal 


plates  separated  by  a  layer,  C,  of  some  insulating  material. 
If  now  these  plates  are  connected  to  the  terminals  of  a  dyna- 
mo they  become  electrostatically  charged.  The  electrostatic 
stress  tends  to  draw  the  plates  together,  and  in  addition  sets 
up  intense  strains  in  the  dielectric  C,  rendering  potential 
thereby  a  certain  amount  of  energy  which  flows  into  the 
apparatus  in  the  form  of  electric  current.  This  energy  is 
returned  as  current  if  the  original  electromotive  stress  is 
removed  and  A  and  B  are  connected  together.  The  medium 
behaves  just  as  if  it  were  a  strained  spring,  and  when  it  returns 
its  energy  to  the  circuit  it  does  so  spring-fashion  with  rapid 
oscillations,  dying  out  the  more  slowly  the  less  resistance  they 
encounter. 

The  capacity  of  such  a  condenser  is  the  quantity  of  energy 
which  it  can  store  up  as  electrostatic  strains  in  C.  It  is  pro- 
portional to  the  area  of  the  plates,  to  the  E.  M.  F.  produc- 
ing the  strains,  and  to  the  "dielectric  constant"  of  C,  that  is, 
the  coefficient  which  for  that  particular  substance  measiu*es  its 


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144  ELECTRIC  TRANSMISSION  OF  POWER. 

power  to  take  up  electrostatic  strains.  Oddly  enough  the 
capacity  decreases  as  C  grows  thicker,  indicating  that  the 
intensity  of  the  strain  is  the  thing  which  counts  rather  than 
the  volume  of  dielectric.  Without  knowing  the  exact  character 
of  electrostatic  strain,  it  is  difficult  to  get  a  clear  mechanical 
idea  of  the  state  of  things  which  causes  the  energy  stored  to 
increase  as  the  thickness  of  C  diminishes.  A  similar  condition, 
however,  holds  for  a  wire  held  tightly  at  one  end  and  twisted 
at  the  other;  the  shorter  the  \\ire,  the  more  energy  stored  for 
a  given  angle  of  twist. 

As  in  the  case  of  inductance,  for  practical  purposes  the  unit 
of  capacity  is  taken  in  terms  of  unit  pressure,  t.e.,  one  volt. 
Unit  capacity,  then,  in  terms  of  energy,  is  the  capacity  of  con- 
denser in  which  one  watt-second  can  be  stored  under  an  elec- 


FIO.  58. 

tromotive  stress  of  one  volt.  This  capacity  is  one  farad,  and 
as  it  is  many  thousand  times  larger  than  anything  found  in 

practice, of  it  (the  microfarad)  is  more  often  used. 

^  '  1,000,000  ^ 

When  a  condenser  is  used  with  an  alternating  current,  the 
rate  at  which  energj'^  is  stored  and  delivered  evidently  increases 
with  the  frequency,  or)  what  is  the  same  thing,  for  a  given 
alternating  E.  M.  F.  the  greater  the  frequency  the  greater  the 
current  received  and  delivered  by  the  condenser. 

Numerically  the  current  in  a  condenser  of  capacity  k  farads, 
supplied  by  an  E.  M.  F.  of  e  volts  at  n  cycles  per  second,  is 

C  =  2  wnek  J 

which  is  simply  the  current  due  to  e  volts  and  k  farads  mul- 
tiplied by  the  frequency  expressed  in  angular  measure.  Thus, 
if  we  have  a  2  microfarad  condenser  fed  by  an  alternating 


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PROPERTIES  OP  ALTERNATING  CIRCUITS.  145 

E.  M.  F.  of  2,000  volts  and  130  cycles  per  second,  the  current 

flowing  is 

^      2,000  X  6.28  X  130  X  2       ^ 

C  = -—^- =  3.26  amperes. 

In  such  an  alternating  circuit,  then,  there  will  be  a  substantial 

current  flowing  in  spite  of  the  fact  that  there  is  a  break  in  the 

conductor  at  the  condenser.     In  short,  the  circuit  acts  as  if  it 

2,000 
had  a  resistance  of  -f— —  =613  ohms,  which  is  the  impedance  of 
3.26  '  ^ 

the  circuit.     More  exactly  the  impedance  is •     It  should 

2  7r  n  k 

be  noted  here  that  some  writers  refer  to  this  fundamental 
condenser  function  (2  w  n)  k  sls  capacity-speed.  Capacity-im- 
pedance really  is  a  negative  reactancej  often  termed  conden^ance. 

To  see  the  relation  which  this  capacity-impedance  bears  to 
other  impedances  in  the  circuit,  it  is  necessary  to  look  into 
the  properties  of  the  E.  M.  F.  of  the  condenser.  As  energy  is 
stored  in  the  condenser  the  opposing  stresses  in  it  increjise 
until  the  applied  E.  M.  F.  can  no  longer  force  current  into  it 
and  the  condenser  is  fully  charged.  At  the  moment,  then, 
when  current  ceases  to  flow,  the  E.  M.  F.  of  the  condenser 
tending  to  discharge  it  is  at  a  maximum.  Hence,  since  the 
one  has  a  maximum  as  the  other  is  zero,  the  E.  M.  F.  of  the 
condenser  and  the  charging  current  are  90°  apart  iji  phase. 

But  the  inductive  E.  M.  F.  is  also  90°  from  the  current,  and, 
as  we  have  seen,  lagging.  It  has  its  maximum  when  the 
current  is  rary^ing  niost  rapidly;  and  when  the  strejigth  of  cur- 
rent in  a  given  direction  is  increasing,  the  inductive  E.  M.  F. 
in  the  same  direction  is  diminishing,  as  shown  in  Fig.  53.  As 
regards  capacity,  however,  the  moment  of  maximum  condenser 
E.  M.  F.  in  a  given  direction  is  that  at  which  the  current 
thereby  becomes  zero,  so  that  as  the  current  changes  sign  it 
has  behind  it  the  thrust  of  the  full  E.  M.  F.  of  the  discharging 
condenser,  while  at  the  same  moment,  as  we  have  just  seen, 
the  opposing  inductive  E.  M.  F.  is  at  its  maximum.  Hence  the 
E.  M.  F.  of  the  condenser  has  a  maximum  in  one  direction 
when  the  inductive  E.  M.  F.  has  its  maximum  in  the  other 
direction.    The  two  are  thus  180°  apart  in  phase,  and  each 


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146 


ELECTRIC  TRANSMISSION  OF  POWER. 


being  90°  from  the  current,  the  condenser  E.  M.  F.  must  be 
regarded  as  90°  ahead  of  the  current,  just  as  the  inductive 
E.  M.  F.  is  90°  behind  it. 

The  condition  of  affairs  is  sho^\'n  in  Fig.  59.  Here  a  a  is 
the  line  of  zero  current  and  E.  M.  F.  All  quantities  above 
this  line  may  be  regarded  as  -I  ,  and  all  below  it  as  —  ;  6  is  a  + 

b 


Fio.  69. 

wave  of  current  to  which  appertains  c  c  the  curve  of  inductive 
E.  M.  F.  lagging  90°  behind  the  current,  and  d  d  the  condenser 
E.  M.  F.,  leading  the  current  90°. 

It  is  evident  that  these  two  E.  M.  F.'s  always  are  opposing 
each  other  —  when  one  is  retarding  the  current  the  other  is 
accelerating  it,  and  vice  versa. 

The  condenser  E.  M.  F.  has  no  effect  on    the  total  energy 


no.  60. 

of  the  circuit  for  the  same  reason  that  held  good  in  respect 
to  Fig.  53;  it  is  obviously  akin  to  a  spring,  alternately  receiv- 
ing and  giving  up  energy,  but  absorbing  next  to  none. 

Capacity  may  be  considered  as  negative  inductance  in  many 
of  its  properties.     If,  as  in  Fig.  59,  it  is  in  amount  exactly 


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PROPERTIES  OF  ALTERNATING  CIRCUITS. 


147 


equivalent  to  the  inductance,  the  total  effect  on  the  circuit  is 
as  if  neither  capacity  nor  inductance  were  in  the  circuit.  In 
such  case  it  is  as  if  C  /?,  Fig.  51,  should  be  reduced  to  zero. 
The  impressed  E.  M.  F.  then  becomes  equal  to  the  effective 
E.  M.  F.,  the  angle  of  lag  vanishes  and  the  circuit  behaves  as 
if  it  contained  resistance  only.  If  the  condenser  E.  M.  F.  is 
not  quite  large  enough  to  annul  the  inductance  it  simply 
reduces  it. 

Fig.  60  illustrates  the  effect  of  varying  amounts  of  capa- 
city. In  the  main  triangle  ABC,  the  sides  have  the  same 
signification  as  in  Fig.  51.       Since  the  capacity  E.  M,  F.  is 


180°  from,  i.e.,  directly  opposite  to,  the  inductive  E.  M.  F.,  the 
effect  of  adding  the  capacity  E.  M.  F.  C  D,  is  to  reduce  the 
effective  inductance  to  B  D  and  give  as  an  impressed  E.  M.  F. 
A  D  and  an  angle  of  lag  <^i.  Now  increasing  C  D  to  equal  C  B, 
the  inductance  is  annulled,  <^  becomes  zero,  and  the  impressed 
and  effective  E.  M.  F.'s  are  the  same.  Then  increase  C  D  still 
further  so  that  it  becomes  C  E,  Now  the  inductance  C  B 
not  only  is  neutralized  but  is  replaced  by  a  negative  inductance 
B  E.  The  angle  of  lag  now  becomes  an  angle  of  lead,  <^2»  the 
necessary  impressed  E.  M.  F.  rises  to  A  E,  and  the  circuit 
behaves  as  regards  the  relations  between  current,  E.  M.  F.,  and 
energy,  just  as  it  did  when  affected  by  inductance.  There  is 
the  same  discrepancy  between  real  and  apparent  energy,  the 
same  necessity  for  more  current  to  represent  the  same  energy. 
But  adding  inductance  now  decreases  the  angle  of  lead.  From 
a  practical  standpoint  capacity  by  itself  is  objectionable,  but 
capacity  in  a  line  containing  inductance  is  sometimes  a  very 
material  advantage. 


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148  ELECTRIC  TRANSMISSION  OF  POWER. 

The  nature  and  reality  of  this  curious  phenomenon  of 
"leading"  current  in  an  alternating  circuit  may  be  appre- 
ciated by  an  examination  of  Fig.  61.  This  shows  the  actual 
curves  of  current  and  E.  M.  F.  taken  from  a  dynamo  working 
on  a  condenser  in  parallel  with  inductance.  The  maximum  of 
the  current  wave  is  very  obviously  in  advance  of  the  maximum 
of  the  E.  M.  F.  wave,  though  by  a  rather  small  amoimt  (ac- 
tually about  6°).  The  capacity  in  this  case  was  between  2  and 
3  microfarads. 

Treating  capacity  as  a  negative  inductance  enables  us  to 
compute  its  effects  quite  easily.  We  have  already  seen  how 
to  reckon  the  impedance  of  a  condenser;  using  the  word  im- 
pedance here  in  its  proper  sense  of  apparent  resistance  by 
whatever  caused.  This  quantity  we  can  add  geometrically  to 
the  ohmic  resistance  of  a  circuit  and  obtain  the  net  impedance 
just  as  in  Fig.  54.  We  must  bear  in  mind,  however,  that  the 
capacity  E.  M.  F.  is  180°  from  the  inductance  E.  M.  F.,  though 
each  is  at  right  angles  to  the  effective  E.  M.  F.  which  is  con- 
cerned with  the  ohmic  resistance. 

Instead,    then,    of     computing     the     total     impedance    as 

Vr^  +  P^  it  becomes  y  r*  +  (^ r)  j  the  second  t^rm  under 

the  radical  being  the  square  of  the  apparent  resistance  due  to 
the  capacity,  just  as  P  expressed  the  square  of  the  apparent 
resistance  due  to  inductance. 

Suppose,  for  example,  we  have  a  resistance  of  100  ohms  in 
series  with  a  condenser  of  4  microfarads  capacity.  The 
impressed  E.  M.  F.  is  2,000  volts  at  130  cycles  per  second. 
What  is  the  total  impedance,  the  resulting  current,  and  the 
angle  <^,  in  this  case  an  angle  of  lead?    Here 

,       6.28  X  130  X  4       __^^ 

2vnk  = =  .003266 

1,000,000 

— ~  -  =  306. 
2vnk 

Laying  off  the  resistance  A  B  in  Fig.  62  as  in  Fig.  54,  and 

drawing to  the  same  scale  at  right  angles  (downward  to 

2  vnk 


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PROPERTIES  OF  ALTERNATING  CIRCUITS. 


149 


emphasize  its  opposition  to  the  inductance  of  Fig.  54),  we 
have  for  the  length  of  the  diagonal  A  C,  which  represents  the 
total  impedance,  VlO(p  +  306^  =  322  ohms.  The  current 
flowing  is  then,  6.21  amperes.     The  angle  ^  is  determined  as 

before  by  tan  <^  ""  Too  ""  ^'^^'  whence  <l>  =  72°,  cos  <^  =  .309, 

so  that  we  are  dealing  with  a  "power  factor"  like  that  pro- 
duced by  a  heavy  inductance,  although  the  current  leads  the 
E.  M.  F.  instead  of  lagging  behind  it.  If  we  consider  an 
inductance  in  series  with  this  circuit,  we  should  have  to  reckon 


FlO.  82. 


it  upward  in  Fig.  62,  thereby  subtracting  it  from  the  former 
length  B  C. 

Suppose  for  example  for  the  given  inductance  L  =  .3  heniy. 
Then  /  =  2  Trn L  =  245.  If  in  Fig.  62  we  draw  245  on  the 
scale  already  taken,  upward  from  C,  we  shall  reach  the  point 
D,     B  D  therefore  is  61,  and  A  Z),  the  resulting  impedance. 

The  new 


is    Viocp  ^  6P  =  117  ohms. 

2  000 
fore   [^_   =  17.09,  and  as  tan  i>^ 


current  is    there- 
61,  <l>,  =  31°.5,  being  still 


117 

an  angle  of  lead. 

It  is  easy  to  see  that  for  a  certain  value  of  7,  the  capacity 
effect  and  inductance  effect  would  exactly  balance  each  other. 


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150  ELECTRIC  TRANSMISSION  OF  POWER. 

This  value  is  obviously  2  «•  n  L  = >    since  then  in  Fig. 

2irnk 

62,  JB  C  —  C  D  =  0,  and  the  impedance  and  resistance  are 

the  same  thing,  while  ^  becomes  zero. 

In  actual  circuits  the  capacity  is  seldom  in  series  with  the 

inductance.     It  is  usually  made  up  of  the  aggregated  capacity 

of  the  line  wires  with  air  as  the  dielectric,  the  capacity  of 

any  underground  cables  that  may  be  in  circuit,  and  finally 

the  capacity  of  the  apparatus,  transformers,  motors,  and  the 

like,  that  may  be  in  circuit.     Generally  the  major  part  of  the 

total  inductance  is  in  the  apparatus  rather  than  the  line,  and 

hence  in  parallel  with  the  capacity.     In  many  cases  nearly 

all  the  inductance  and  capacity  is  due  to  the  apparatus,  and 

the  two  may  be  regarded  as  in  parallel  substantially  at  the 


S 

X 

_  ^ 

Fio.  63. 

ends  of  the  line.  The  inductance  of  generators  and  trans- 
formers may  amount  to  several  henrys,  while  their  capacity  is 
by  no  means  small,  though  very  variable,  like  the  inductance. 
For  example,  the  capacity  of  a  large  high-voltage  generator  or 
transformer  may  often  amount  to  several  tenths  of  a  micro- 
farad. Armored  or  sheathed  cable  has  a  capacity  of  from  a 
quarter  to  a  half  or  more,  microfarad  per  mile.  Altogether 
one  may  expect  to  find  a  capacity  of  several  microfarads 
frequently,  and  large  fractions  of  a  microfarad  very  often. 

Suppose  now  we  have  in  parallel  a  capacity  A ,  Fig.  63,  of  2 
microfarads,  and  an  inductance  of  .5  henrys,  the  resistance  con- 
nected with  each  being  insignificant.  Assuming  as  before  2,000 
volts  and  130  cycles,  what  is  the  total  impedance  of  the  com- 
bination, and  the  resulting  current?  We  have  already  seen 
how  impedances  in  parallel  are  to  bo  treated.     In  the  case  in 

hand  the  impedance  of  A  is  — ^^    ^^^  =613  ohms, 

*  6.28  X  130  X  .000,002 

and  that  of  B  Is  6.28  X  130  x  .5  =  408  ohms.    Now  remem- 


fel 

o 

A              B° 

a 

O 

C3 

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PROPERTIES  OF  ALTERNATING  CIRCUITS, 


151 


bering  that  in  adding  impedances  their  geometrical  sum  is  to 
be  taken,  and  that  joint  impedance  is  the  reciprocal  of  the 
geometrical  sum  of  the  reciprocals  of  its  components,  we  can 
proceed  as  follows  :  The  reciprocal  of  613  is  .00163.  This  we 
will  lay  off  to  any  convenient  scale  just  as  in  Fig.  57.  As  it  is 
capacity-impedance,  we  will  draw  it  downward  for  the  sake  of 
uniformity,  making  A  J5,  Fig.  64.  Now  take  the  inductance. 
The  reciprocal  of  408  is  .00245.  As  the  inductance  and  capa- 
city E.  M.  F.'s  are  here  as  before  at  an  angle  of  180°,  we  must 
draw  this  upward  from  B,  giving  us  the  distance  B  C.  The 
geometrical  sum  is  then  A  C  =   .00082,  of  which  the  recip- 


B 
Fia.  64. 


rocal  gives  the  resultant  impedance  as  1,219  ohms.     Hence 

2  000 
the  net  current  in  the  line  under  2,000  volts  is    '        =1.6 

ampere.  But  under  the  same  pressure  the  current  in  A  would 
obviously  be  3.26  amperes  and  that  in  the  inductance  B  would 
be  4.90  amperes.  We  have  then  the  curious  phenomenon  of 
a  total  current  in  the  line  smaller  than  that  through  either  of 
the  two  impedances  in  circuit.  It  is  as  if  A  and  B  formed  a 
local  circuit  by  themselves  in  which  the  condenser  A  served 
as  a  species  of  generator.  It  is  quite  evident  that  the  total 
energy  of  the  system,  however,  is  that  due  to  the  current  in 
the  line,  so  that  the  phases  in  A  and  B  are  greatly  displaced. 
If  the  resistances  in  the  circuit  were  quite  negligible,  the  net 
current  in  the  line  would  be  indefinitely  small  when  A  =  B, 


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152  ELECTRIC  TRANSMISSION  OF  POWER. 

that  is,  when  L  =  -  •    Of  course,  however,  the  true  impedances 

K 

of  both  A  and  B  are  modified  by  the  resistances,  however  small, 
so  that  in  Fig.  64  the  impedances  will  always  be  at  a  small  angle 
with  the  E.  M.  F.'s  instead  of  being  coincident.  Hence  the  net 
current  can  never  become  zero,  though  when  the  impedances 
of  A  and  B  are  large  compared  with  the  resistances,  the  line 

current  will  be  very  small  when  L  =  -  • 

K 

This  case  is  in  sharp  contrast  to  that  in  which  condenser 
and  inductance  are  in  series  with  each  other.     For  then  the 

line  current  is  increased  as  L  approaches  -  instead  of  becom- 

ing  smaller  relatively  to  the  branch  currents,  although  in  each 
case  the  same  relation  between  capacity  and  inductance  gives 
the  maximum  "power  factor"  on  the  circuit,  since  whatever 
the  current,  under  this  condition  it  depends  most  nearly  on  the 
resistance  alone.  When  the  resistance  is  quite  perceptible  in 
comparison  with  the  impedances  of  A  and  B,  we  should  form  a 
'resultant  impedance  with  each,  and  then  combine  the  two 
somewhat  as  in  Fig.  56. 

If  then  we  have  an  inductive  load  of  any  kind  in  circuit,  a 
condenser  in  parallel  therewith  will  reduce  the  current  on  the 
line  and  thereby  increase  the  "power  factor*'  of  the  system. 
It  does  this,  too,  without  any  material  loss  of  energy  and  with- 
out necessarily  increasing  the  amount  of  current  flowing  through 
the  inductance  under  a  given  E.  M.  F.  on  the  line.  Were 
condenser  and  inductance  in  series,  the  power  factor  could 
likewise  be  improved  up  to  a  certain  point,  but  trouble  would 
be  encountered  in  that  the  condenser  would  necessarily  have 
to  be  large  enough  to  let  pass  enough  current  to  supply  the 
energy  required  in  the  inductance  at  full  load. 

In  all  practical  cases  the  relations  between  resistance, 
capacity,  and  inductance,  which  have  just  been  set  forth,  are 
somewhat  modified  by  the  existence  of  losses  of  energy  in  the 
circuit  quite  apart  from  these  due  merely  to  overcoming  of 
resistance.  Energy  is  required  to  reverse  the  magnetization 
of  the  iron  cores  of  inductance  coils,  and  to  reverse  the  electric 


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PROPERTIES  OF  ALTERNATING  CIRCUITS. 


158 


strains  in  the  dielectric  of  condensers.  It  therefore  happens 
that  with  a  condenser  in  circuit  its  apparent  current  is  not 
exactly  90°  ahead  of  its  impressed  E.  M.  F.,  as  shown  in  Fig. 
59,  but  a  trifle  less,  so  that  the  current  has  a  small  component 
in  phase  with  its  E.  M.  F.,  thus  supplying  the  energy  in  ques- 
tion. The  deviation  from  90°  is  generally  but  a  small  fraction 
of  a  degree.  The  same  sort  of  thing  happens  when  an  induc- 
tance having  an  iron  core  is  in  circuit.  However  small  the 
resistance,  the  lag  still  misses  90°  by  enough  to  take  accoimt 
of  the  energy  required  for  magnetic  losses.  The  variation 
from  90°  in  this  case  may  amount  to  30°  or  more.     Hence 


RESISTANCE 


the  failure  to  take  accoimt  of  these  energy  losses  in  the  exam- 
ple given  on  page  141. 

The  result  is  that  in  adding  inductance  and  capacity  effects, 
one  sometimes  seems  not  to  get  so  simple  results  as  in  Fig.  64, 
but  something  more  like  Fig.  65.  Here  it  is  clear  that  no  com- 
bination of  capacity  and  inductance  can  leave  the  circuit  free 
from  everything  except  resistance,  for  both  the  inductance  and 
the  capacity  demand  energy  in  the  circuit  beyond  that  ex- 
pended in  the  resistance.  Evidently,  however,  ^  may  be 
reduced  to  zero  if  the  relation  between  capacity  and  induc- 
tance is  just  right.  Thus  while  the  lag  may  be  reduced  to 
zero,  no  combination  can  dodge  the  energy  losses.  Whenever 
all  the  energy  losses  are  taken  into  account,  the  true  induc- 
tance and  capacity  E.  M.  F.'s  will  be  found  180°  apart  and 
90**  from  the  energy,  exactly  where  they  belong. 

Closely  connected  with  this  subject  is  the  matter  of  reso- 


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154 


ELECTRIC  TRANSMISSION  OF  POWER, 


nance,  which  will  be  taken  up  in  connection  with  the  discus- 
sion of  the  line.  Briefly  the  phenomenon  is  this:  We  have 
seen  that  the  E.  M.  F.  of  a  condenser  is  a  maximum  when  the 
current  is  zero,  so  that  as  the  current  changes  sign  the  thrust 
of  the  condenser  E.  M.  F.  is  behind  it.  Now  if  the  condenser 
E.  M.  F.  synchronizes  with  this  current,  the  impressed  E.  M.  F. 
is  added  to  it,  imposes  an  added  stress  on  the  condenser  dur- 
ing the  next  alternation,  catches  therefrom  an  additional  kick 
as  it  passes  through  zero  again,  and  so  on.  Thus  the  net 
effective  E.  M.  F.  Is  raised  by  the  action  of  the  condenser, 
and  would  increase  enormously  but  for  its  being  frittered 
away  in   overcoming   resistance   and   supplying  such  energy 


Fig.  66. 

losses  as  we  have  just  been  considering.  By  avoiding  these 
losses  as  far  as  possible,  one  can  actually  raise  the  voltage  on 
an  alternating  circuit  to  twenty-five  or  thirty  times  its  nominal 
amoimt  by  employing  a  condenser  of  the  proper  capacity. 
Even  when  the  impressed  E.  M.  F.  and  the  current  are  not 
quite  in  phase,  one  has  always  a  component  of  the  condenser 
E.  M.  F.  tending  to  act  in  a  similar  manner.  Whether  it 
actually  produces  a  sensible  rise  of  voltage  depends  on  its  rela- 
tions to  the  frequency  and  resistance  with  which  it  has  to  deal. 
In  fact,  it  is  the  addition  of  this  same  condenser  E.  M.  F.  to 
the  circuit  that  enables  one  to  neutralize  inductive  E.  M.  F. 
Whether  or  not  the  neutralization  of  inductance  by  capacity 
produces  a  real  resonant  rise  of  voltage  depends  on  the  fre- 


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PROPERTIES  OF  ALTERNATING  CIRCUITS.  165 

quency  and  whether  the  energy  losses  are  small  or  large.  If 
they  are  small  enough  to  let  the  sum  of  the  impressed  and  the 
condenser  E.  M.  F.  accumulate  during  several  alternations 
there  will  be  a  noticeable  increase  of  voltage,  otherwise  not. 

The  dynamics  of  rcsonance  may  perhaps  be  best  understood 
by  a  very  pretty  mechanical  analogue  due  to  Dr.  Pupin.  The 
apparatus  on  which  it  is  based  is  shown  in  Fig.  66.  It  is  a 
torsional  pendulum  composed  of  a  heavy  bar  A  suspended  by 
a  sHjf  elastic  wire  B,  from  a  light  circular  bearing  plate  C. 
This  plate  rests  in  a  recess  a,  with  a  frictional  resistance  which 
can  be  regulated  by  the  screw  showTi  in  the  cut.  Such  an 
apparatus  acts  much  like  an  electric  circuit,  having  induc- 
tance, capacity,  and  ohmic  resistance.  The  moment  of  inertia 
of  the  bar  A  corresponds  to  self-induction,  the  elasticity  of 
B  to  condenser  capacity  as  we  have  just  noted  in  connection 
with  Fig.  58,  and  the  friction  of  C  to  the  resistance.  More- 
over, if  /  is  the  moment  of  inertia  of  the  bar  A,  and  B  the 
reciprocal  of  the  elastic  capacity  of  the  wire,  then  within  cer- 
tain values  of  the  frictional  resistance  the  oscillation  period  of 
the  pendulum  thus  formed  is,  in  seconds, 

7^  =  2^  VTb, 

This  correspcmds  most  beautifully  to  the  time  constant  of  an 
electric  circuit,  which  is,  if  the  energy  losses  are  within  cer- 
tain limits, 

1,000 

wherein  L  is  in  henrys,  C  the  capacity  in  microfarads,  and 
the  denominator  comes  from  the  units  being  thus  chosen. 

Now,  if  this  pendulum  be  given  a  twist  it  will  oscillate  at 
constant  frequency  imtil  the  friction  gradually  brings  it  to 
rest  with  oscillations  of  steadily  decreasing  amplitude.  If, 
however,  at  the  end  of  each  complete  swing  it  should  receive  a 
slight  push,  its  oscillations  would  continue  and  would  increase 
in  amplitude  up  to  a  limit  set  by  the  frictional  resistance. 
The  condition  for  such  permanent  increase  of  amplitude  is 
that  the  frequency  of  the  pushes  must  coincide  with  the  period 
of  the   pendulum.    In   the   electrical   case,   resonance    thus 


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156  ELECTRIC  TRANSMISSION  OF  POWER, 

occurs  when  the  frequency  and  the  time  constant  of  the  cir- 
cuit are  equal.  Further,  maintaining  our  auxiliary  pushes  at 
their  original  frequency,  suppose  /  to  be  decreased  by  taking 
weight  off  A  progressively.  As  the  time  constant  of  the 
pendulum  thus  diminished  a  point  would  be  found,  and  that 
very  soon,  at  which  resonance  would  cease,  and  the  same 
result  would  follow  increase  of  /,  so  that  when  the  circuit 
begins  to  get  out  of  tune  the  resonance  soon  becomes  rather 
trivial.  If,  however,  the  pushes  were  supplemented  by  others 
of  3,  5,  7,  etc.,  times  the  frequency,  corresponding  to  the 
harmonics  found  in  an  ordinary  alternating  circuit,  new  points 
of  resonance  would  appear  when  the  period  of  A  assumed 
corresponding  values. 

As  to  the  magnitude  of  the  resonant  effect,  in  the  torsional 
pendulum  case  the  amplitude  evidently  increases  with  the 
strength  of  the  pushes,  their  absolute  frequency,  which  mea- 
sures the  energy  supplied,  and  the  moment  of  inertia  of  A^ 
which  stores  this  energy.  It  decreases  in  virtue  of  the  fric- 
tional  resistance.  Corresponding  reasoning  holds  in  the  elec- 
trical case,  and  to  a  first  approximation  the  E.  M.  F.  in  a 
completely  resonant  circuit  is 

in  which  E  is  the  impressed  E.  M.  F.  concerned,  L  the  induc- 
tance in  henrys,  R  the  resistance  in  ohms,  and  n  the  frequency. 
In  case  of  resonance  with  harmonics,  n  and  E  refer  to  the 
frequency  and  magnitude  of  the  harmonic  implicated,  and  E' 
becomes  a  resonant  component  of  the  E.  M.  F.  wave.  This 
subject  will  be  discussed  more  at  length  in  Chapter  XIII. 

We  have  now  glanced  at  the  most  striking  characteristics 
of  alternating  currents  —  those  concerned  with  the  phenomena 
of  inductance  and  capacity. 

It  rcmains  to  note  very  briefly  some  other  physical  proper- 
ties that  are  of  practical  importance. 

The  most  important  single  property  of  alternating  current 
is  the  ease  with  which  it  can  be  changed  inductively  from  one 
voltage  to  another.  If  a  circuit  carrying  such  a  current  is  put 
in  inductive  relation  with  another  circuit  as  in  Fig.  4,  Chapter 


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PROPERTIES  OF  ALTERNATING  CIRCUITS.  157 

I,  the  electro-magnetic  stresses  set  up  by  the  first  circuit  can  be 
utilized  to  produce  alternating  current  of  any  desired  voltage 
in  the  second  circuit.  The  details  of  the  operation  will  be 
taken  up  later;  suffice  it  to  say  here  that  it  is  essentially  the 
transformation  of  the  electro-magnetic  energy  due  to  one 
circuit  into  electrical  energy  in  another  circuit. 

Alternating  currents  can  be  regulated  in  amount  by  putting 
inductance  in  the  circuit  without  losing  more  than  a  very 
trifling  amount  of  energy.  This  very  property,  however,  is 
troublesome  when  an  alternating  current  is  used  for  magnetiz- 
ing purposes.  It  is  very  difficult  to  get  a  large  current  to  flow 
around  a  magnet  core  because  of  the  high  inductance,  and  even 
then  the  magnetic  and  other  losses  in  the  core  are  serious  imless 
great  care  is  taken.  These  difficulties  have  stood  in  the  way 
of  getting  a  good  alternating  motor  until  within  the  past  few 
years,  and  even  now  such  motors  have  to  be  designed  and  con- 
structed with  the  greatest  care  to  avoid  trouble  from  induc- 
tance and  iron  losses.  For  some  classes  of  work,  such  as  teleg- 
raphy and  electrolytic  operations,  the  alternating  current  is 
ill  suited  save  under  special  conditions  and  with  special  appar- 
atus. For  the  general  purposes  of  electrical  power  transmis- 
sion it  is  singularly  well  fitted,  from  the  great  ease  with  which 
transfonnations  of  voltage  can  be  made,  certain  very  valuable 
properties  of  the  modem  alternating  motor,  and  the  great 
simplicity  and  efficiency  with  which  regulation  can  be  efifected. 
The  only  inconvenience  attending  transmission  by  alter- 
nating current  is  that  incurred  when  direct  current  must  for 
one  reason,  or  another,  be  supplied.  This  is  in  a  fair  way  to 
be  greatly  reduced  by  both  increasing  use  of  alternating  cur- 
rent in  distribution,  and  by  improvement  in  apparatus  for 
obtaining  direct  current  from  an  alternating  source. 


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CHAPTER  V. 

POWER    TRANSMISSION    BY    ALTERNATING    CURRENTS. 

Broadly  considered,  we  may  say  that  all  systems  of  trans- 
mitting power  by  alternating  currents  are  closely  akin  in 
principles  and  characteristics.  The  growth  of  the  art,  how- 
ever, has  proceeded  along  several  lines,  and  certain  conven- 
tional distinctions  have  come  to  be  observed  in  considering 
the  methods  employed  for  rendering  the  alternating  current 
applicable  to  the  working  conditions  of  power  transmission. 

Alternating  systems  are  usually  classified  as  either  mono- 
phase or  polyphase.  By  the  former  term  is  generally  imder- 
stood  a  system  generating,  transmitting,  and  utilizing  a  simple 
alternating  current  such  as  shown  in  diagram  in  Fig.  44.  By 
the  latter  is  meant  a  system  generating,  transmitting,  and 
utilizing  two  or  more  such  currents  differing  in  phase  and 
combined  in  various  ways.  As  regards  the  systems,  this  dis- 
tinction is  sufficiently  sharp,  but  as  regards  individual  parts  of 
such  systems  the  line  of  demarcation  is  sometimes  hazy,  since 
a  monophase  current  may  be  the  source  of  derived  polyphase 
currents,  and  on  the  other  hand  polyphase  currents  may  be  so 
combined  as  to  give  a  monophase  resultant.  Mixed  systems 
involving  unsymmetrical  phase  relations  may  properly  be 
called  heterophase. 

As  regards  apparatus,  any  device  that  performs  all  its  func- 
tions in  a  normal  manner  when  deriving  all  its  energy  from  a 
simple  alternating  current  should  be  classified  as  monophase. 
If  its  functions  require  the  cooperation  of  energy  received 
from  two  or  more  alternating  currents  differing  in  phase, 
the  apparatus  is  essentially  polyphase. 

For  certain  purposes  the  one  system  is  best  adapted,  for 
certain  other  purposes  the  other  is  most  advantageous,  but 
the  underlying  principles  are  the  same,  and  the  apparatus  has 
much  the  same  general  properties. 

The  material  of  alternating  transmission  work  may  be  classi- 

158 


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TRAXS\fISSIOX  BY  ALTERXATIXG  CURREXTS.      159 

fied  as  follows,  the  transmission  line  itself  being  reserved  for 
discussion  in  a  separate  chapter  in  connection  with  other  line 
work: 

I.   Generators.  III.   Synchronous  Motors. 

II.   Transformers.  IV.   Induction  Motors. 

In  addition  to  these,  there  have  been  recently  introduced 
alternating  series-wound  motors  with  commutators  which  will 
be  discussed  in  their  proper  place. 

After  a  tolerably  careful  examination  of  the  practical  prop- 
erties of  this  apparatus  in  its  various  forms,we  shall  be. able 
to  appreciate  its  application  to  the  electrical  transmission  of 
power  under  various  circumstances.  Subsidiary  apparatus  of 
all  kinds  will  be  referred  to  elsewhere,  and  the  divers  systems 
that  have  been  exploited  can  best  be  considered  after  we 
have  looked  into  the  characteristics  of  their  component 
parts. 

Alternating  power  transmission  is  now  going  through  the 
stage  of  development  that  is  inseparable  from  the  rise  of  a 
comparatively  new  art  —  the  planting  time  of  "systems,"  if 
one  may  be  allowed  the  simile.  It  is  sufficiently  certain 
alread}''  that  the  same  sort  of  plant  will  not  do  equally  well 
under  all  circumstances. 

The  principles  of  the  alternating  current  dynamo  have 
already  been  explained,  but  the  constructional  features  of  such 
machines  are  sufficiently  distinct  from  those  of  continuous 
current  dynamos  to  warrant  examination  in  considerable 
detail. 

The  modifications  peculiar  to  alternators  are  in  general  due 
to  two  causes ;  first,  the  general  use  of  a  fairly  high  frequency, 
and,  second,  the  necessities  of  rather  high  voltage. 

We  have  already  seen  that,  while  an  ordinary  continuous 
current  dynamo  fitted  with  collecting  rings  will  give  alternat- 
ing current,  the  frequency  is  rather  low.  To  secure  a  higher 
frequency  it  becomes  necessary  to  increase  the  number  of  poles, 
the  speed,  or  both.  Increasing  the  number  of  the  poles  is 
the  usual  method  employed,  since  continuous  current  dynamos 
are  generally  for  the  sake  of  keeping  up  the  output  operated  at 
speeds  as  high  as  the  conditions  of  economical  use  render 
desirable.    So  we  usually  find  that  for  equal  outputs  alternators 


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160 


ELECTRIC  TRANSMISSION  OF  POWER. 


have  many  more  poles, 
speed,  and  frequency  is, 


The  general  relation  between  poles, 


w  =  -  TT 


2  60 


where  p  is  the  number  of  poles,  A"  the  revolutions  per  min- 
ute, and  n  the  complete  cycles  per  second. 

For  example,  belt-driven  continuous  current  dynamos  of  100 
to  500  kilowatts  usually  run  at  speeds  from  600  down  to  300, 
and  have  four  or  six  poles,  thus  giving  15  to  20  cycles  per  sec- 


Fio.  67. 


ond,  while  modem  alternators  of  similar  size  and  speed  have 
from  12  to  24  poles,  thus  adapting  them  for  a  frequency  of 
30^  to  60 '^.  Machines  for  the  older  frequencies  of  120^  to 
140 '^  were  usually  even  more  liberally  provided  with  poles 
unless  driven  at  speeds  considerably  above  those  mentioned. 
The  general  appearance  and  design  of  a  typical  belted  alter- 
nator is  shown  in  outline  in  Fig.  67.  This  is  a  150  KW  gener- 
ator running  at  600  revolutions  per  minute,  and  shows  admir- 
ably the  general  characteristics  of  rather  numerous  poles,  low 


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fttAMMlSSION  BY  ALTERNATING  CURRENTS,     l6l 

base,  and  massive  bearings  that  nowadays  belong  in  common 
to  machines  by  nearly  all  makers.  Such  alternators  usually 
have  very  powerful  field  magnets,  and  the  projecting  pole- 
pieces  are  usually  built  up  of  iron  plates  Uke  the  armature,  for 
the  same  purpose  of  preventing  eddy  currents  in  the  iron.  The 
ring  of  field  magnets  is  split  on  the  level  of  the  centre  of  the 
shaft,  for  convenience  in  removing  the  armature.    The  weight 


Feo.  68. 

of  belt-driven  generators  of  the  output  named  is  usually  six 
or  seven  tons. 

This  same  general  type  is  commonly  adhered  to  whatever 
the  nature  or  voltage  of  the  armature  winding,  save  in  the 
case  of  special  machines. 

The  winding  of  a  modem  alternator  is  nearly  always  widely 
dififerent  from  continuous-current  windings.  In  alternators  the 
voltage  is  generally  from  1,000  volts  up,  seldom  below  500  volts, 
and  to  obtain  this  the  windings  corresponding  to  the  numerous 
poles  are  almost  universally  connected  in  series  instead  of  in 
parallel. 

This  necessitates  connecting  the  numerous  armature  coils 
in  a  very  characteristic  way.     For  when  a  given  armature 


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162  ELECTRIC  TRANSMISSION  OF  POWER. 

coil  is  approaching  one  of  the  north  poles  of  the  field  magnet 
and  is  generating  current  in  a  given  direction,  the  next  arma- 
ture coil  is  necessarily  approaching  the  neighboring  south  pole, 
and  if  woimd  in  the  same  direction  as  the  first  coil  would 
generate  a  current  flowing  in  the  opposite  direction.  Hence 
if  all  the  armature  coils  are  to  be  in  series,  they  must  be 
wound  alternately  in  opposite  directions,  as  shown  in  Fig.  68. 
This  arrangement  throws  in  series  the  E.  M.  F.'s  generated 
by  all  the  armature  coils.  Sometimes  for  convenience  the 
halves  of  the  armature  are  connected  in  parallel,  thus  giving 
half  the  voltage  and  twice  the  current  by  a  simple  change  in 
connections.     Fig.  69  shows  in  diagram  such  a  winding  for  a 


Fio.  69. 

16-pole  field,  and  its  relation  to  the  collecting  rings.  Note 
that  each  half  of  the  winding  preserves  the  characteristics 
shown  in  Fig.  68. 

In  practical  machines  as  built  to-day,  the  armature  coils  are 
nearly  always  bedded  in  slots  in  the  armature  core.  The 
early  American  machines  were  generally  built  with  smooth 
armature  cores,  and  upon  these  flat  coils  were  laid  and  held  in 
place  by  an  elaborate  system  of  binding  wires.  This  construc- 
tion has  been  virtually  abandoned  by  all  the  principal  manu- 
facturers in  favor  of  the  so-called  "iron-clad"  armature,  which 
has  the  double  advantage  of  great  mechanical  solidity  and  of 
permitting  the  armature  coils  to  be  wound  in  forms  thoroughly 
insulated,   and   then  dropped  into  place  in  their  slots  and 


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TRANSMISSION^  BY  ALTERNATING  CURRENTS.     163 


firmly  wedged  in  position.  The  winding  is,  therefore,  very 
little  liable  to  damage  and  easily  replaced  if  necessary. 

The  slotted  armature  cores  are  variously  arranged  in  dif- 
ferent macnhies,  but  always  with  the  same  object  in  view. 

Fig.  70  shows  one  widely  used  arrangement  of  slots.  Here 
the  coils  are  wound  in  forms  and  thoroughly  insulated.  They 
are  then  pushed  into  place  in  the  previously  insulated  slot, 
each  coil  enclosing  a  single  armature  tooth.     When  firmly  in 


FiQ.  70. 


place  the  insulating  material  is  put  into  position  above  them 
and  a  hard-wood  wedge  is  driven  into  the  dove-tailed  upper 
portion  of  the  slot,  holding  the  coils  and  their  surrounding 
insulation  permanently  in  place.  The  coils  here  shown  con- 
sist of  only  four  turns  of  heavy  wire.  Often  there  are  many 
more  turns  per  coil,  and  frequently  the  round  wire  is  replaced 
by  rectangular  bars.     In  generators  for  use  with  raising  trans- 


FiQ.  71. 


formers  each  coil  sometimes  consists  of  a  single  turn  of  bar 
copper,  but  whatever  the  nature  of   the   coil  the   slots   are 
.  arranged  much  as  here  shown. 

Another  familiar  form  of  slotted  armature  is  shown  in 
Fig.  71.  The  coils  are,  as  in  the  case  just  mentioned,  woimd 
in  forms  and  solidly  insulated.  They  are  then  sprung  over 
the  armature  teeth  into  place  and  tightly  wedged.  The 
slots  are  carefully  insulated  also,  and  by  the  time  the  winding 
is  completely  assembled  it  is  so  thoroughly  insulated  that 
repairs  are  few  and  far  between.    The  special  peculiarity  of 


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164  ELECTRIC  TRANSMISSION  OF  POWER. 

this  form  of  core  is  that  the  outer  comers  of  the  teeth  are  cut 
away,  so  that  the  coils  come  more  gradually  into  the  field  of 
the  pole-pieces  than  if  the  edges  were  sharp.  The  object  of 
this  device  is  to  obtain  a  curve  of  E.  M.  F.  more  nearly  accord- 
ing with  the  sine  wave  form,  and  experience  shows  that  the 
plan  works  successfully.  Without  such  precautions  the  E.  M.  F. 
curve  is  very  likely  to  be  quite  irregular,  and  even  with 
them  it  is  generally  none  too  smooth.  The  pole-pieces  of 
alternators  are  very  often  similarly  rounded  off  or  chamfered 
away  for  the  same  purpose. 

Nearly  all  modem  alternating  windings  are  like  those  just 
indicated,  of  the  drum  type.  The  Gramme  winding  is  seldom 
or  never  employed,  as  it  is  hard  to  wind  and  repair  and  has, 
for  alternators,  no  compensating  advantages.  Nor  has  the 
flat  coil  \\inding  without  iron  core  found  a  permanent  place  in 
American  practice,  although  it  is  somewhat  used  abroad. 
There  is  considerable  likelihood  of  eddy  currents  in  the  arma- 
ture conductors  of  such  machines  unless  they  are  indi\'idually 
very  thin,  and  for  this  and  obvious  mechanical  reasons  Ameri- 
can designers  have  adhered  to  the  iron-clad  armature,  which 
is  admirable  mechanically  and  magnetically,  and  have  taken 
other  means  to  escape  the  difficulty  of  its  high  ijiductance. 

As  in  other  dynamos,  the  theoretical  E.  M.  F.  generated  by 
an  alternator  depends  on  the  strength  of  the  magnetic  field, 
the  number  of  armature  conductors  under  induction,  and 
the  speed  at  which  they  are  driven  through  the  field.  As  an 
altcmator  receives  load  the  p].  M.  F.  at  its  terminals  is  reduced 
by  three  several  causes. 

First,  there  is  a  loss  of  voltage  due  to  energy  lost  in  the 
armature  conductors.  This  depends  simply  on  the  current 
and  resistance  and  is  numerically  equal  to  C  R. 

Second,  there  is  self-induction  in  the  armature  windings, ' 
which,  as  we  have  already  seen,  involves  an  inductive  E.  M.  F., 
lagging  90®  behind  the  impressed  E.  M.  F.  The  effect  of 
this  is  to  partly  neutralize  the  impressed  E.  ^f.  F.,  as  in  all 
cases  of  inductance.  The  amount  of  this  disturbance  depends 
on  the  frequency  and  the  magnetic  relation  of  the  armature 
coils  to  each  other  and  to  the  field  magnets.  This  relation 
of  course  varies  according  to  the  relative  position  of  the  arma- 


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TRANSMISSION  BY  ALTERNATING  CURRENTS.     166 

ture  teeth  which  carry  the  coils.  In  Fig.  72,  purposely  shown 
with  somewhat  exaggerated  teeth,  the  armature  is  in  the 
position  of  minimum  inductance,  for  the  magnetic  field  set 
up  by  the  armature  coils  is  not  here  much  strengthened  by 
the  presence  of  the  pole-pieces.  If,  however,  the  armature 
were  shifted  forward  or  backward  so  that  each  tooth  would 
be  just  opposite  a  pole-piece,  the  field  from  the  armature 
coils  would  traverse  an  almost  complete  loop  of  iron  and  the 
inductance  of  the  armature  would  be  a  maximum.  In  this 
position  the  armature  teeth  might  be  almost  as  good  magnet 
poles  as  the  field  poles  themselves;  at  all  events,  consecutive 


Fio.  72. 

teeth  would  be  united  by  an  almost  continuous  iron  core,  and 
the  armature  inductance  would  be  very  high. 

One  of  the  best  ways  of  reducing  this  inductance  and  its 
train  of  troubles  is  to  make  the  magnetization  due  to  the  field 
magnets  as  strong  as  is  practicable.  This  not  only  utilizes  the 
iron  of  the  field  magnets  and  armature  to  the  best  advantage, 
but,  so  to  speak,  preempts  its  power  of  receiving  magnetiza- 
tion so  that  the  current  about  the  armature  teeth  finds  a  poor 
field  for  its  inductive  operations.  In  addition,  this  strengthen- 
ing of  the  field  enables  the  required  E.  M.  F.  to  be  obtained 
with  fewer  turns  per  tooth.  This  of  itself  is  a  great  advan- 
tage, since  increasing  the  number  of  turns  in  an  iron-cored 
coil  runs  up  the  inductance  with  appalling  rapidity.  A  glance 
at  Fig.  73  will  show  the  reason  why.  Suppose  we  have  a 
looped  iron  core  wound  with  four  turns  of  vire,  a,  6,  c,  d.  If 
we  pass  a  certain  alternating  current  around  two  turns,  a  and 


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166 


ELECTRIC  TRANSMISSION  OF  POWER. 


hj  we  shall  have  a  certain  inductance  due  to  the  reaction  of 
the  change  in  magnetism  on  these  two  coils.  Now,  pass  the 
same  current  around  all  four  coils.  The  magnetization  will 
be  approximately  doubled  and  the  number  of  turns  on  which 
it  acts  will  also  be  doubled.  That  is,  each  coil  is  acted  upon 
by  double  the  force  and  there  are  twice  as  many  total  coils. 
Hence,  the  total  inductance  will  be  about  four  times  as  great 
as  at  first,  and  in  general  it  will  increase  with  the  square  of 
the  number  of  turns.  If,  however,  as  just  suggested,  the 
core  is  nearly  saturated  already,  adding  the  two  extra  turns, 
c  and  d,  will  not  anywhere  nearly  double  the  magnetization, 


Fig.  73. 

since  iron  already  magnetized  responds  less  and  less  to  addi- 
tional magnetizing  force  as  this  force  increases. 

Hence,  diminishing  the  number  of  armature  turns  that  can 
act  conjointly  in  producing  effective  magnetization  lowers  the 
inductance  very  rapidly. 

The  third  disturbing  cause  which  tends  to  reduce  the 
effective  E.  M.  F.  of  an  alternator  is  the  reaction  of  the 
armature  current,  through  the  resulting  magnetization,  on 
the  field  magnets.  We  have  already  seen  that  when  a  closed 
coil  is  driven  into  and  out  of  a  magnetic  field  the  induced 
current  is  always  in  such  direction  as  to  cause  work  to  be 
done  in  driving  the  coil.  But,  since  the  current  due  to  enter- 
ing the  field  is  equal  and  opposite  to  that  produced  in  leaving 


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TRANSMISSION  BY  ALTERNATING  CURRENTS.     167 

the  field,  the  total  mafi^etizations  due  to  these  currents  are 
equal  and  opposite,  and  if  one  opposes  the  field  due  to  a  pole- 
piece  the  other  will  in  an  equal  degree  strengthen  that  field. 
Hence,  provided  these  two  actions  are  applied  alike;  i.e., 
are  symmetrical  with  respect  to  the  field,  the  total  effect  of 
armature  current  will  be  neither  to  weaken  nor  strengthen 
the  field. 

In  practice  the  effect  of  the  armature  reaction  is  two-fold. 
If  the  current  be  nearly  in  phase  with  the  E.  M.  F.  the  main 
result  of  the  magnetic  field  set  up  by  the  armature  is  to 


Fig.  74. 

distort  that  due  to  the  field  without  greatly  weakening  it  as 
a  whole.  The  result  of  this  distortion  is  that  the  E.  M.  F. 
does  not  increase  and  decrease  steadily  following  a  sine  wave, 
but  becomes  irregular.  The  working  E.  M.  F.,  as  measured 
on  a  voltmeter,  changes  but  a  trifle,  but  the  maximum 
E.  M.  F.  becomes  subject  to  great  variations.  Fig.  74  shows 
in  a  very  striking  manner  the  result  of  field  distortion  from 
a  purely  non-inductive  load.  Here  a  is  the  E.  M.  F.  curve 
on  open  circuit  and  b  is  the  curve  as  modified  by  the  armature 
reaction  at  nearly  full  load.  The  arrow  shows  the  direction 
of  rotation   of   the   armature.     In   this   case   the   maximum 


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168  ELECTRIC  TRANSMISSION  OF  POWER. 

voltage  was  jncreased  about  30  per  cent,  while  the  measured 
voltage  was  nearly  constant.  Bearing  in  mind  that  the 
E.  M.  F.  at  any  moment  is  due  to  the  rate  of  change  of  the 
magnetic  induction  through  the  armature,  and  not  to  the 
absolute  amount  of  that  induction,  it  is  tolerably  obvious  that 
the  effect  of  field  distortion  due  to  armature  reaction  may 
vary  widely  according  to  the  shape  and  position  of  both 
the  pole-pieces  and  the  armature  teeth.  It  may  increase  the 
maximum  voltage  as  above,  or  decrease  it  fully  as  much,  but 
if  it  is  of  any  considerable  magnitude  it  always  deforms  the 
E.  M.  F.  wave  very  materially. 

If,  however,  through  armature  inductance  or  inductive  load 
the  current  lags  behind  the  E.  M.  F.,  we  have  a  very  different 
state  of  affairs.  The  current  reaches  its  maximum  after  the 
armature  coil  has  passed  beyond  the  position  of  maximum 
E.  M.  F.,  and  the  net  magnetization  produced  by  it  chokes 
back  the  field,  at  the  same  time  greatly  distorting  it. 

If  the  only  effect  of  armature  reaction  and  inductance  were 
to  cause  a  loss  of  voltage  there  would  be  little  cause  for  alarm. 
But  as  shown  in  Fig.  74,  the  E.  M.  F.  wave-shape  often  un- 
dergoes profound  changes,  which  may  greatly  increase  the 
chance  for  serious  resonance.  As  already  noted,  alternating 
generators,  monophase  and  polyphase  alike,  give  in  practice  an 
E.  M.  F.  wave  which  is  not  sinusoidal,  but  contains  the  odd 
harmonics  of  the  fundamental  frequency.  These  are  a  neces- 
sary result  of  the  variations  in  magnetic  reluctance  and  arma- 
ture reactance  when  the  armature  is  in  various  angular 
positions,  as  well  as  of  subsidiary  reactions  in  transformers 
and  other  apparatus.  The  harmonics  of  even  order  do  not 
appear,  since,  unless  a  machine  is  deliberately  made  unsym- 
metrical,  all  the  variations  in  E.  M.  F.  are  complete  within 
each  half  period,  the  second  half  of  the  cycle  merely  showing 
a  reversal  of  sign.  Hence,  only  those  harmonics  appear  which 
are  themselves  symmetrical  with  respect  to  a  half  period  of 
the  fundamental,  i.e.,  by  construction  all  the  harmonics  are 
of  odd  order.  These  harmonics  have  a  very  real  existence, 
and  can  readily  be  identified  by  testing  electrically  for  reso- 
nance, or  even  by  hunting  for  them  with  a  telephone  in  some 
cases.     By  taking  the  wave  form  of  the  machine  by  the  con- 


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TRANSMISSION  BY  ALTERNATING  CURRENTS.     169 

tact  method  or  photographically,  the  nature  and  magnitude  of 
the  harmonics  are  at  once  made  evident. 

Fig.  75  shows  the  wave  form  of  a  machine  that  was  carefully 
studied  by  Steinmetz.  It  is  from  a  three-phase  generator  hav- 
ing but  one  armature  tooth  per  phase  per  pole,  and  giving  150 
KW  at  2,000  volte  and  60  -^.  Curve  A  is  the  E.  M.  F.  wave  of 
one  coil  to  the  common  connection,  at  no  load,  B  is  the  wave 
as  calculated  from  a  summation  of  the  harmonics  up  to  the 
fifteenth,  and  C  shows  the  residual  traces  of  still  higher  har- 
monics.   To  reduce  the  vertical  scale  to  primary  volte,  mul- 


Z             7  X         i  X 

*                   7     *■             ^1 

111                        I-         K         -A 

.                        U         ^v^/ 

!           X                   t 

;        4              X 

m                   -3                                              i 

I         M                        K- 

l          M^                          \ 

m                    ^                                                                             .        \ 

;     t                  5^ 

I  ^                    s^ 

*i'  J  J     U  '10'  fe'  m     1^    m^mm    &  w    »'  m  m  m'm   d 

FlQ.  76. 

tiply  by  10.  Analysis  of  this  wave  showed  that  it  corre- 
sponded approximately  to  the  following  equation : 

Sin  a  -  .12  sin  (3a  -  2.3)  -  .23  sin  (5a  -  1.5) 
+  .134  sin  (7a  -  6.2). 

In  other  words  the  third  harmonic  has  about  12  per  cent,  the 
fifth  about  23  per  cent,  and  the  seventh  about  13  per  cent  of 
the  amplitude  of  the  fundamental. 

At  full  load  the  shape  of  this  wave  is  changed  in  a  most  sin- 
gular manner.  The  armature  reaction  shifte  the  magnitudes 
and  positions  of  the  variations  in  the  magnetic  field  and  of  the 
harmonics  due  to  them.     Fig.  76  shows  the  wave  form  from 


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170 


ELECTRIC  TRANSMISSION  OF  POWER. 


this  machine  under  load.  The  central  depression  of  Fig.  75 
is  replaced  by  a  slight  hollow  between  a  high  peak  and  a  shoul- 
der, and  the  wave  is  conspicuously  unsymmetrical,  as  might 
readily  be  predicted  from  the  general  effect  of  the  armature 
reaction.    The  approximate  equation  to  the  wave  of  Fig.  76  is 

Sin  a  -  .176  sin  (3a  +  11.7)  -  .085  sin  (5a  -  33.8) 
+  .01  sin  (7a  -f  26.6). 

The  effect  of  the  armature  reaction  due  to  load  has  been 
greatly  to  strengthen  the  third  harmonic,  greatly  to  weaken 
the  fifth,  and  nearly  to  suppress  the  seventh. 


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Fia.  76. 

Obviously  changes  of  this  sort  may  have  a  very  great  effect 
in  the  matter  of  resonance.  Suppose,  for  example,  that  the 
conditions  on  the  line  at  light  load  were  such  as  to  give 
marked  resonance  with  the  seventh  harmonic  of  the  frequency. 
Now,  imder  all  ordinary  working  conditions  this  harmonic 
would  be  practically  absent;  but  if  a  large  part  of  the  load  were 
thrown  off,  resonance  would  suddenly  appear,  and  with  the 
lessened  armature  reaction  the  general  voltage  would  rise 
sharply,  so  that  serious  results  might  follow.  In  case  of  a  high 
voltage  generator,  say  for  10,000  volts,  having  the  curves  just 
given,  at  load  the  seventh  harmonic  would  only  have  an  ampli- 


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TRANSMISSION  BY  ALTERNATING  CURRENTS.     171 

tude  of  about  100  volts,  while  this  amplitude  would  suddenly 
rise  to  1,340  volts,  increased  perhaps  four  or  five  times  by 
resonance,  when  the  load  was  thrown  off.  Under  other  con- 
ditions throwing  on  load  might  produce  an  equally  unpleasant 
effect. 

Lest  it  should  be  suppi^sed  that  these  wave  distortions  with 
the  presence  of  strong  high  harmonics  are  extraordinary  and 


Fio.  77. 

of  merely  theoretical  importance,  examples  of  such  action 
from  recent  machines  of  first  class  make,  obtained  in  commer- 
cial service,  are  here  given.  Fig.  77  shows  the  E.  M.  F.  curve 
from  a  750  KW,  5,500  volt  engine-driven  three-phaser,  dis- 
torted by  the  presence  of  a  strong  thirteenth  harmonic.  The 
generator  had  a  monodontal  winding  which  is  prone  to  give 
lower  harmonics,  but  these  higher  ones  were  mainly  due  to 


Fio.  78. 

the  disturbing  effect  of  a  synchronous  motor  load.  Fig.  78 
shows  E.  M.  F.  and  current  waves  from  a  1,500  KW  three- 
phase  turbo-generator,  also  on  a  synchronous  motor  load,  and 
displaying  conspicuous  harmonics  of  the  twenty-third  order, 
in  this  case  not  traceable  to  the  nature  of  the  load,  but  structural 
and  merely  aggravated  by  the  running  conditions.  The  gen- 
erator had  four  slots  per  phase  per  pole  in  this  case,  but  the 
magnetic  density  in  the  teeth  was  rather  low.     Under  ordinary 


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172  ELECTRIC  TRANSMISSION  OF  POWER. 

circuiTistances  these  harmonics  are  probablj'^  quite  harmless, 
but  their  frequency  is  so  great  that  they  might  easily  cause 
serious  results  with  but  a  very  moderate  amount  of  capacity  in 
the  system.  The  curves  shown  are  from  oscillograph  curves, 
reported  by  Dr.  W.  M.  Thornton  to  the  British  Institute 
Electrical  P^ngineers,  and  are  quite  sufficient  to  prove  the  im- 
portance of  the  subject. 

Such  eccentricities  can  be  avoided  by  scrupulous  care  in 
design,  at  least  for  the  most  part,  and  should  be  eliminated 
from  every  machine  used  upon  circuits  where  by  reason  of 
unusual  length,  or  the  presence  of  cables,  there  is  danger  of 
resonant  effects.  At  the  voltages  now  generally  used  for  power 
transmission,  insulation  is  difficult  enough  without  incurring 
the  risks  that  come  from  preventable  dangers  of  this  sort. 

The  magnetizing  and  demagnetizing  effects  of  the  arma- 
ture current  in  case  of  inductive  load  no  longer  can  balance 
each  other,  for  they  are  unsymmetrical  with  respect  to  the 
poles.  If  the  angle  of  lag  is  large  the  result  will  be  a  very 
serious  weakening  of  the  field,  and  a  correspondingly  large 
drop  in  the  effective  voltage.  For  example,  a  certain  alter- 
nator of  120  KW  output  has  40  turns  of  wire  per  armature 
tooth,  carrying  a  normal  full  load  current  of  60  amperes. 
There  is  thus  a  possible  demagnetizing  force  of  2,400  ampere- 
turns  at  full  load.  The  ampere-turns  per  pole-piece  in  the 
same  machine  are  3,600,  so  that  if  the  current  should  lag 
enough  to  give  the  armature  reaction  full  play,  as  might 
happen  from  excessive  armature  inductance  alone,  the  total 
net  magnetizing  force  would  be  reduced  to  a  third  of  its  nor- 
mal amount  and  the  resulting  voltage  to  a  half  or  less.  It 
is  in  fact  common  enough  to  find  alternators  that  require 
from  50  to  100  per  cent  increase  in  the  exciting  ampere-turns 
to  hold  them  at  normal  voltage  under  a  full-load  current 
lagging  even  15°  or  20°. 

Between  inductance  and  armature  reaction  the  effective 
E.  M.  F.  of  alternators  generally  falls  off  rapidly  imder  load 
unless  special  care  be  taken  with  the  design.  The  loss  from 
ohmic  resistance  is  usually  trivial  compared  with  those  just 
named.  It  is,  in  fact,  perfectly  practicable  to  build  an  alterna- 
tor with  inductance  and  armature  reaction  so  exaggerated, 


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TRANSMISSION  BY  ALTERNATING  CURRENTS.     173 

that  a  very  slight  increase  in  current  will  cut  down  the  voltage 
so  rapidly  as  to  keep  the  current  virtually  constant.  This  plan 
was  successfully  carried  out  in  the  remarkable  Stanley  alter- 
nating arc  machine  of  a  few  years  ago. 

In  this  case  the  current  varied  only  about  10  per  cent,  while 
the  voltage  varied  between  a  few  volts  and  over  2,000.  An 
automatic  short-circuiting  switch  was  provided  to  avert  dan- 
gerous rise  of  voltage  in  case  of  an  accidental  open  circuit. 

In  so-called  constant  potential  alternators,  as  usually  built, 
the  inherent  regulation  is  by  no  means  good.  Fig.  79  gives 
an  excellent  idea  of  the  performance  of  some  of  the  earlier 
machines  in  this  respect,  and  it  is  about  what  one  would  find 


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0           2 

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0            6 

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AMPERES 
Fig.  79. 

in  many  alternators  now  in  service,  except  for  their  compound 
winding. 

It  has  often  been  held  that  high  inductance  and  large  arma- 
ture reaction  are  desirable  in  alternators  in  order  to  prevent 
bum-outs  in  case  of  accidental  short  circuits.  While  it  is  per- 
fectly true  that  sufficiently  crude  armature  design  does  produce 
this  effect,  by  limiting  the  possible  current,  it  is  equally  true 
that  a  machine  with  sufficient  inductance  and  reaction  to  serve 
as  a  practical  safeguard  will  regulate  so  atrociously  as  to  be 
imder  many  circumstances  incapable  of  decent  commercial 
service  under  present  conditions.  When  it  was  sufficient  for 
an  alternator  to  give  current  that  with  sufficient  hand  regula- 
tion could  supply  house  to  house  transformers  most  of  the 


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174 


ELECTRIC  TRANSMISSION  OF  POWER. 


time,  high  inductance  machines,  which  are  easy  and  cheap  to 
build,  answered  the  purpose. 

At  present,  when  the  importance  of  good  regulation  is  gener- 
ally understood,  and  most  large  alternating  plants  must  look 
forward  to  assuming  a  motor  load,  low  inductance  machines 
with  small  armature  reaction  are  essential  for  first-class  service. 
For  power  transmission  plants  with  heavy  mixed  loads  of 
lights  and  motors,  no  other  class  of  machine  should  be  toler- 
ated, or  can  be  used  without  incessant  annoyance. 

Most  even  of  the  older  alternaLors  are  compound-wound 
to  compensate  for  armature  effects,  and  are  thus  enabled  to 
work  successfully  up  to  outputs  at  which  the  voltage  begins  to 


i 


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NON: 

INDUCTIVE 

LOAD 

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KILOWATTS  OUTPUT 
Fia.  80. 


fall  off  too  fast  to  be  thus  compensated.  So  long  as  the  com- 
pounding process  actually  gives  good  regulation,  it  is  useful 
and  enables  the  generators  to  be  worked  at  a  high  output.  As 
a  matter  of  fact  when  used  with  generators  of  the  older  type, 
even  compounding  left  much  to  be  desired.  As  alternating 
practice  has  gradually  improved,  compound-wound  alternators 
have  been  more  skillfully  designed,  and  recent  machines  give 
on  non-inductive  load  a  very  fair  approximation  to  constant 
potential.  Fig.  80  shows  the  E.  M.  F.  of  a  modem  over-com- 
pounded alternator  at  varying  load.  If,  however,  the  current 
has  even  a  moderate  lag  behind  the  E.  M.  F..  owing  to  induc- 
tance in  the  machine  or  the  load,  the  machine  will  no  longer  give 
constant  potential,  and  the  voltage  may  fall  off  rapidly  as  the 


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TRANSMISSION  BY  ALTERNATING  CURRENTS.     175 

load  comes  on,  as  shown  in  the  cut.  The  reason  for  this  we 
have  already  found  in  the  extra  increase  of  field  excitation 
necessary  to  compensate  for  the  demagnetizing  effect  of  arma- 
ture reaction.  Incidentally  if  the  current  commuted  to  supply 
the  series  field  lags  much,  the  process  of  commutation  cannot 


Fig.  81. 

go  on  normally  without  adjusting  the  brushes  to  compensate 
for  the  lag. 

Therefore,  for  inductive  load  the  compounding  has  to  be 
greatly  increased,  and  even  then  is  correct  only  for  a  particu- 
lar inductance. 

It  must  be  understood  that  alternators  are  compounded  on 
the  same  general  principles  as  continuous  current  machines, 
except  that  instead  of  the  current  for  the  series  winding  being 
derived  from  the  general  commutator  of  the  dynamo,  it  is 


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176 


ELECTRIC  TkANSMlJSSlON  OF  POWER. 


generally  obtained  from  a  simple  special  commutator.  A 
shunt  around  this  commutator  diverts  most  of  the  main  cur- 
rent, while  a  portion  is  rectified  and  passed  around  the  fields. 
Fig.  81  shows  in  diagram  a  common  compounding  arrange- 
ment. The  two  collecting  rings  A  and  B  with  the  commutator 
C  are  moimted  on  the  armature  shaft.  Brushes  on  A  and  B 
take  off  the  alternating  current.  One  of  these  rings,  A,  leads 
directly  to  line.  The  current  going  to  the  other  ring  is  divided, 
part  passing  aroimd  C  through  the  resistance  box  D,  and  part 
being  rectified  by  the  commutator  for  use  in  the  series  field. 
This  commutator  has  as  many  segments  as  there  are  pairs  of 
poles  in  the  field,  the  alternate  sections  being  electrically, united. 


fii/Vt 

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OUTPUT  IM  K.W 
Fig.  82. 

By  varying  the  resistance  D,  the  amount  of  current  diverted 
into  the  field  can  be  varied,  and  the  compounding  may  thus  be 
arranged  to  keep  the  voltage  constant  at  the  terminals  or  at 
any  point  on  the  line.  A  similar  change  in  D  may  be  made 
to  adjust  the  compounding  for  inductive  load  of  any  given 
power  factor. 

For  non-inductive  loads,  or  for  inductive  loads  of  constant 
power  factor,  this  compounding  gives  good  results,  but  for  a 
load  of  widely  varying  power  factor  it  is  nearly  worthless 
unless  supplemented  by  hand  regulation. 

If  compounding  is  to  be  successfully  used  for  keeping  con- 
stant potential  on  a  circuit  of  lights  and  motors  subject  to 
considerable  variations  in  the  power  factor,  it  must  be  applied 
to  a  generator  of  very  low  inductance  and  armature  reactioa. 


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TRANSMISSION  BY  ALTERNATING  CURR£!NTS.     177 

Otherwise  no  adjustment  of  the  compounding  for  any  particu- 
lar power  factor  will  give  approximately  constant  potential 
when  the  power  factor  varies. 

For  example  it  would  be  hopeless  to  attempt  to  compound 
in  the  ordinary  way  an  alternator  having  a  characteristic 
like  Fig.  79,  so  that  it  would  be  tolerable  on  a  commercial  cir- 
cuit of  lights  and  motors.  On  the  other  hand,  a  generator 
having  a  voltage  characteristic  like  Fig.  82  could  readily  be 
so  compounded.  Here  the  fall  in  voltage  at  constant  field 
excitation,  from  no  load  to  full  load  (non-inductive),  is  about 
3J  per  cent.  Under  inductive  load  this  fall  would  be  in- 
creased considerably,  but  from  the  usual  ratio  of  inductive 
drop  to  armature  reaction  found  in  the  best  modern  gener- 
ators, the  variation  for  the  power  factors  likely  to  be  encoun- 


Fio.  83. 

tered  with  a  mixed  load  would  be  somewhat  smaller  than  the 
original  drop.  The  total  variation .  from  no  load  to  full  in- 
ductive load  would  then  be  between  6  and  7  per  cent,  and 
with  compounding  adroitly  adjusted  for  average  conditions 
the  greatest  variation  from  normal  voltage  could  easily  be 
brought  within  2  per  cent.  A  little  intelligent  hand  regu- 
lation at  certain  times  of  the  day  would  improve  even  this 
good  result. 

These  considerations  apply  to  polyphase  as  well  as  to  mono- 
phase generators.  The  advent  of  polyphase  work  has  done 
much  to  improve  all  alternators,  and  especially  with  respect  to 
regulation. 

The  generation  of  polyphase  alternating  currents  is  a  very 
simple  matter.  The  object  in  view  is  the  production  of  two  or 
more  similar  currents  differing  in  phase  by  some  convenient 


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178  ELECTRIC  TRANSMISSION  OF  POWER. 

amount,  usually  60°  or  90°.  To  obtain  two  currents  90°  apart 
in  phase,  it  is  only  necessary  to  clamp  together  the  shafts  of 
two  common  alternators,  so  that,  for  a  construction  like  Fig. 
70,  the  slots  of  one  armature  would  be  opposite  the  teeth  of 
the  other  armature.  The  armatures  would  then  give  currents 
90°  apart  in  phase.  Sucl^  combination  alternators  were  built 
for  the  Columbian  Exposition  by  the  Westinghouse  Company, 
and  were  used  for  the  principal  lighting  and  power  circuits. 
These  structures  arc,  however,  expensive  for  the  output  obtained, 
and  the  two  windings  arc  nearly  always  put  on  a  single  arma- 
ture core,  and  spaced  as  just  described.  Fig.  83  shows  dia- 
grammatically  a  winding  of  this  nature.     There  are.  four  times 


Fig.  8*. 

as  many  armature  slots  as  there  are  field  poles.  Each  coil 
spans  two  teeth.  The  coils  shown  by  solid  lines  form  one  phase 
winding,  the  dotted  coils  the  other  phase  winding.  Each  set 
of  coils  is  connected  as  an  ordinary  monophase  winding,  and 
the  terminals  are  brought  out  to  two  pairs  of  collecting  rings. 
Such  a  winding  gives  two  simple  alternating  currents  related 
in  phase  as  shown  in  Fig.  84.  The  armature  core  is  very  fully 
occupied  by  the  two  windings,  rather  more  advantageously 
than  it  could  be  by  a  single  winding,  so  that  the  machine  gives 
a  somewhat  better  output  as  a  two-phaser  than  would  be 
possible  with  a  simple  alternator  of  the  same  dimensions. 
And,  what  is  of  more  importance,  the  regulation  of  the  machine 
as  a  two-phaser  is  much  better  than  it  would  be  as  a  single- 
phaser.  In  the  first  place  the  armature  inductance  is  greatly 
reduced  by  the  distribution  of  the  windings  and  the  reduction 
of  the  ampere-turns  per  armature  tooth.     Second,  the  same 


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TRANSMISSION  BY  ALTERNATING  CURRENTS.     179 

causes  act  to  cut  down  the  armature  reaction  in  case  of  a  lagg- 
ing current.  Anything  that  improves  the  intrinsic  regu- 
lation also  means  greater  output  for  unimproved  regulation. 
Moreover,  the  increased  number  of  armature  teeth  gives  a 
more  imiform  reluctance  than  in  the  case  of  fewer  teeth,  and 
hence  tends  to  give  a  better  approximation  to  a  sinusoidal 
wave  form. 
So,  aside  from  the  value  of  polyphase  currents  for  motor  pur- 


FlG.  8S. 

poses,  which  we  shall  presently  examine,  polyphase  winding  is 
valuable  on  its  own  accoiuit  as  increasing  output  and  improv- 
ing regulation.  In  fact,  diphase  wandings  were  devised  for  this 
purpose  before  their  importance  in  the  operation  of  motors 
became  generally  known. 

The  value  of  a  subdivided  winding  in  reducing  inductance 
and  armature  reaction  was  greatly  emphasized  by  the  intro- 
duction of  polyphase  generators,  and  it  was  a  short  step  from 
monodonotal  windings  having  one  coil  and  virtually  one  tooth 
per  phase  per  pole,  to  windings  in  which  each  phase  winding  is 
split  up  into  several  sets  of  coils  in  adjacent  slots,  thereby 
still  further  decreasing  the  effective  inductance  and  armature 
reaction.  Such  windings  may  be  called  polyodontaly  from  their 
several  teeth  per  phase  per  pole,  and  are  very  generally  used  in 


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180  ELECTRIC  TRANSMISSION  OF  POWER. 

the  best  recent  machines.  A  fine  example  of  this  class  of 
winding  is  shown  in  Fig.  85.  This  is  a  quarter  section  of  the 
armature  of  one  of  the  original  5,000  HP  Niagara  generators, 
showing  a  portion  of  one  coil  belonging  to  a  single  phase. 
The  full  winding  is  composed  of  two  conductors  per  slot,  half 
the  total  slots,  in  alternate  groups,  belonging  to  each  phase. 

Such  complete  subdivision  of  the  coils  results  in  low  induc- 
tance and  a  very  low  armature  reaction.  A  similar  winding 
could  be  used  for  a  monophase  generator,  and  will  have  to  be 
employed  if  monophase  machines  come  to  be  used  extensively 
for  power  transmission  purposes.  The  form  of  armature  slot 
used  for  polyodontal  windings  is  shown  in  Fig.  86,  a  single 
segment  of  one  of  the  core  plates  of  the  armature  of  the  Niagara 
two-phaser.    The  appearance  of  one  of  these  great  machines 


Fio.  86. 


complete  is  admirably  shown  in  the  frontispiece,  showing  the 
interior  of  the  Niagara  station.  The  field  magnets  are  re- 
volved instead  of  the  armature,  although  they  are  exterior  to 
it.  A  very  pow^erful  fly-wheel  effect  is  gained  by  this  arrange- 
ment, since  the  weight  of  the  revolving  structure,  turning  at 
250  r.  p.  m.,  is  about  75  tons,  half  of  this  being  in  the  field 
itself.  This  is  about  12  feet  in  diameter,  a  single  forged  steel 
ring  with  twelve  massive  pole-pieces  secured  to  its  inner  face. 
The  normal  voltage  of  the  machine  is  about  2,250,  and  the 
frequency  is  25^.  The  stationary  armature  is  provided  with 
six  ample  ventilating  ducts,  through  which  air  is  forced  by 
the  revolving  field.  Fig.  87  shows  a  vertical  section  of  the 
whole  apparatus  with  its  shaft  and  upper  bearings.  A  hun- 
dred and  forty  feet  below  the  generator  is  the  turbine  which  sup- 
ports by  hydraulic  pressure  the  weight  of  the  revolving  mass, 


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TRANSMISSION  BY  ALTERNATING  CURRENTS.      181 

save  a  ton  or  two  of  residual  weight,  which  may  be  either  posi- 
tive or  negative,  and  which  is  taken  care  of  by  a  thrust  bearing. 
The  full  load  of  this  generator  is  775  amperes  on  each  of  the 
two  circuits,  and  at  this  load  the  commercial  efficiency  is 
nearly  97  per  cent  —  a  figure  very  close  to  the  possible  max- 
imum. The  exciting  current  for  the  fields  is  derived  from  a 
rotary  transfonner,  and  is  led  into  the  revolving  magnets 
through  a  pair  of  collecting  rings  shown  in  Fig.  87  at    the 


Fio.  87. 


extreme  top  of  the  shaft.  The  armature  current  is  of  course 
taken  from  stationary  binding  posts.  Altogether  this  Niagara 
machine  was  a  fine  specimen  of  polyphase  construction. 

When  three-phase  currents  instead  of  two-phase  are  to  be 
generated,  separate  armatures  are  out  of  the  question,  and  a 
winding  similar  to  that  of  Fig.  83  is  frequently  employed.  To 
obtain  the  three  currents,  however,  three  separate  windings 
are  employed,  arranged  as  in  Fig.  88.    The  coils  are  connected 


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182 


ELECTRIC  TRANSMISSION  OF  POWER. 


SO  that  a,  a,  a,  etc.,  form  one  phase  winding,  6,  6,  etc.,  a  second, 
and  Cy  c,  etc.,  the  third.  The  close  similarity  of  this  winding 
to  the  two-phase  shown  in  Fig.  83  is  at  once  apparent. 

It  is  worth  noting  that  these  three  windings  are  spaced  60® 
apart,  instead  of  90°,  as  in  a  winding  for  two  phases.  Natur- 
ally, therefore,  the  currents  generated  would  be  different  in 


Fio.  88. 

phase  by  only  60®,  giving  the  arrangement  of  currents  shown 
in  Fig.  89.  This  is  homologous  with  the  two-phase  current 
system  of  Fig.  84. 

In  practice   it    is    necessary,  however,  to  have  the  sym- 
metrical arrangement  of  phases  given  by  three  similar  cur- 


Fio.  89. 

rents  120°  apart.  This  is  very  easily  obtained  in  the  external 
circuit  by  winding  one  set  of  the  armature  coils  in  a  direc- 
tion reversed  from  the  other  two,  or  by  merely  reversing  the 
termmals  in  making  connections.  The  result  of  this  is  a  true 
three-phase  current,  such  as  is  shown  in  diagram  in  Fig.  90. 


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TRANSMISSION  BY  ALTERNATING  CURRENTS.     183 

It  has  now  the  curious  property  that  at  all  times  the  system  is 
simultaneously  carrying  currents  substantially  equal  in  both 
directions,  as  will  readily  appear  from  inspection  of  the  curves. 
With  such  a  current  it  is  usual  to  combine  the  circuits  cor- 
responding to  the  several  armature  windings.  Otherwise  we 
would  be  compelled  to  deal  with  circuits  of  six  wires,  and  the 
generator  would  have  six  collecting  rings. 

Moreover,  the  distribution  circuits  formed  by  combining 
the  circuits  as  just  indicated  have  the  advantage  of  economy 
in  copper,  as  we  shall  presently  see.  Hence,  the  three-phase 
system  has  become  the  mainstay  of  electrical  power  transmis- 
sion so  far  as  the  principal  circuit  is  concerned.     The  genera- 


Fio.  1)0. 

tors  may  be  two-phase  and  the  distributing  circuits  two-phase 
when  convenience  dictates,  but  the  main  line  is,  save  in  very 
rare  instances,  worked  three-phase.  The  change  from  two- 
phase  to  three-phase,  or  the  reverse,  is  accomplished  in  a  beau- 
tifully simple  and  efficient  manner,  to  be  described  later. 
Under  certain  circumstances  the  use  of  a  two-phase  generator 
has  at  least  the  theoretical  advantage  that  the  currents  in 
the  respective  armature  windings,  being  in  quadrature,  can 
have  little  or  no  mutual  reaction,  so  that  the  two  phases  are 
more  independent  than  the  three  phases  of  a  three-phaser. 

As  might  be  expected,  the  subdivision  of  windings  in  a  three- 
phase  armature  results  in  small  inductance  and  armature  reac- 
tion, smaller  in  fact  than  would  be  found  in  a  similar  two-phase 
winding.  Nevertheless,  experience  shows  that  if  the  annature 
has  only  a  single  coil  per  phase  per  pole,  the  reaction  is  too 
great  for  first-class  regulation,  and  the  curve  of  E.  M.  F.  is 


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184 


ELECTRIC  TRANSMISSION  OF  POWER, 


rather  too  vAAq  a  departure  from  the  sine  wave.  It  is  quite 
usual,  therefore,  to  adopt  the  polyodontal  construction  with 
from  two  to  four  coils  per  phase  per  pole.  A  machine  carefully 
designed  on  these  lines  can  be  made  to  give  excellent  regulation, 
with  voltage  not  varying  more  than  3  or  4  per  cent  from  no 
load  to  full  non-inductive  load,  and  is  capable  of  giving  a  very 
close  approximation  to  a  true  sinusoidal  wave,  a  valuable 
characteristic  for  longidistance  transmission.  Fig.  91  shows 
the  wave  form  giv^n  by  one  of  these  polyodontal  three-phasers. 


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FlO.  91. 

The  full  curve  shows  the  actual  E.  M.  F.,  the  dotted  line  the 
corresponduig  sine  curve,  and  the  irregular  line  at  the  base  of 
the  figure  the  difference  between  the  two. 

There  are  several  methods  of  connecting  a  three-phase  wind- 
ing to  its  external  circuit.  The  two  chiefly  used  are  generally 
known  as  the  "star''  and  "mesh''  connections.  In  the  former, 
one  end  of  each  of  the  three  windings  is  brought  to  a  common 
jimction,  and  the  three  remaining  ends  are  connected  to  three 
line  wires.    The  three  Unes  then  serve  in  turn  as  outgoing  and 


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TRANSMISSION  BY  ALTERNATING  CURRENTS.     185 

return  circuits,  the  maximum  current  shifting  in  regiilar  rota- 
tion from  one  to  the  others  in  succession.  The  three  E.  M.  F/s 
in  the  three  coils  differ  in  phase  by  120°,  owing  to  the  reversal 
of  which  we  have  spoken.  We  may  draw  the  star  connection 
diagrammatically  in  Fig.  92,  drawing  the  three  coils  ah  c  120® 
apart  to  show  the  relation  of  the  E.  M.  F.'s  and  currents, 

A 


Fig.  92. 

although  they  lie  on  the  armature  as  shown  in  Fig.  88.  Three 
of  the  terminals  meet  at  the  point  o,  the  others  are  connected 
respectively  to  the  lines  A,  B^  C.  As  the  three  windings  on  the 
armature  are  alike,  the  E.  M.  F.'s  generated  by  the  three  coils 
are  equal.  So  if  each  winding  a,  6,  c,  is  designed  for  1,000 
volts,  that  will  be  the  voltage  between  the  point  o  and  each  of 
the  three  lines  A,  B,  C,    Clearly,  however,  the  voltage  between 


Fio.  93. 

any  two  of  these  lines,  as  A  and  B,  is  a  very  different  matter, 
since  it  results  from  the  addition  of  the  voltages  of  a  and  6, 
which  are,  however,  120°  apart  in  phase.  They  must  then  be 
added  geometrically.  Now  the  chord  of  120°  is  V3  times  the 
radius,  so  that  the  geometrical  sum  of  the  voltages  a  and  6, 
120°  apart,  is  1.732  times  either  of  them.    The  voltages  then 


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186  ELECTRIC  TRANSMISSION  OF  POWER. 

between  A  and  B  in  the  case  in  hand  aggregate  1,732.  The 
same  is  evidently  true  of  the  other  pairs  of  lines  B,  C,  and  C,  A, 

The  other  ordinary  three-phase  connection  is  the  mesh,  in 
which  the  six  terminals  of  the  three  coils  are  united  two  and 
two,  and  the  lines  are  connected  to  the  three  points  of  junc- 
tion. This  arrangement  is  shown  diagrammatically  in  Fig.  93. 
Here  each  coil  must  generate  the  full  E.  M.  F.  between  any 
two  of  the  lines,  but  the  current  in  any  line,  as  B,  is  made  up  of 
the  geometrical  sum  of  the  currents  in  a  and  6,  differing  in 
phase,  just  as  the  E.  M.  F.  between  lines  in  Fig.  92  was  made 
up  of  the  sum  of  two  E.  M.  F.'s.  The  current  inB  being  then 
so  constituted,  is  V3  times  the  current  in  a  or  6,  and  so  on  for 
the  other  lines.  In  the  mesh  connection  we  deal  with  resul- 
tant currents  just  as  in  the  star  we  find  resultant  E.  M.  F.'s. 

An  armature  designed  for  a  given  working  voltage,  measured 
in  the  ordinary  way  between  lines,  would,  if  planned  for  star 
connection,  have  fewer  turns  of  larger  wire  than  if  intended  for 
mesh  connection.  This  is  sometimes  convenient,  and  is  useful 
in  keeping  the  voltage  between  coils  low.  The  mesh  connec- 
tion on  the  other  hand  has  more  turns  of  smaller  wire,  as  the 
current  is  diminished  while  the  E.  M.  F.  in  each  coil  is  the  full 
E.  M.  F.  between  lines.  This  property  is  useful  under  certain 
conditions,  as  it  makes  the  E.  M.  F.  between  any  two  lines 
somewhat  less  dependent  on  the  actions  going  on  in  the  other 
pairs  of  lines.  The  same  windings  can  of  course  be  connected 
either  star  or  mesh,  according  to  the  dictates  of  convenience. 
Both  these  combination  circuits  have  in  common  one  immensely 
valuable  property.  They  require  for  the  transmission  of  a 
given  amount  of  energy  at  a  given  percentage  of  loss,  only 
75  per  cent  of  the  weight  of  copper  reqyired  for  the  same  trans- 
mission at  the  same  working  voltage,  by  continuous  current 
or  by  any  alternating  system  having  two  wires  per  phase. 
That  is,  if  100  tons  of  copper  are  required  for  a  given  transmis- 
sion by  continuous  current,  single-phase  alternating,  two- 
phase  with  two  circuits,  or  three-phase  with  three  circuits, 
75  tons  will  suffice  for  the  same  transmission  by  the  star  or 
mesh  three-phase  circuit  without  any  increased  loss  of  energy. 
The  proof  of  this  saving  is  very  simple.  Assume  a  three-phase 
circuit  carrying  a  non-inductive  load  at  V  volts  between  lines, 


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TRANSMISSIOX  BY  ALTERNATING  CURRENTS.     187 

the  current  in  each  line  being  /  and  the  resistance  r.  Then  for 
a  star  connection,  as  we  have  already  seen,  the  voltage     in 

each  branch  to  the  neutral  point  o  (Fig.  92)  is  V — --r,  the  cur- 

V3 

rent  in  each  branch  is  /,  the  power  in  each  branch  is— ^,-    IV, 

V3 

and  the  total  power  is  IV   ^S. 

The  loss  in  each  branch  of  the  circuit  is  obviously  Pr,  and 

the  total  loss  for  the  above  power  3/V.     Now  let  the  same 

amount  of  power  be  transmitted  by  a  single-phase  circuit  at 

the  same  voltage  V,     The  current  will  evidently  have  to  be 

/  v3.    Let  r'  be  the  resistance  of  one  of  the  two  monophase 

wires,  such  that  the  total  loss  shall  be  3Pr  as  before.     The 

resistance  of  the  complete  circuit  will  be  2-/,  and  the  total  loss 

6P/.     But  since 

6Pr'  =  3Pr, 

2 

That  is,  the  resistance  of  each  of  the  monophase  wires  must  be 
only  one-half  the  resistance  of  a  single  three-phase  wire.  The 
cross  section  of  each  monophase  wire  must  then  be  double  the 
cross  section  of  one  three-phase  wire.  If  the  weight  of  the  lat- 
ter be  w,  the  total  weight  of  the  three-phase  copper  will  be 
SWy  while  the  weight  of  th^  two  monophase  leads  of  double 
cross  section  will  evidently  be  4w  for  a  circuit  of  the  same 
length.  A  mesh  connected  three-phase  system  leads  to  ex- 
actly the  same  result,  since  tlie  voltage  in  each  branch  is  V 

(see  Fig.  93),  the  current  is — —  ,    the  power  per  branch  — r/F, 

V3  V3 

the  total  power  IV  V3,  and  the  loss  3Pr,  as  before. 

The  result  seems  so  singular  that  in  the  early  days  of  the 
three-phase  system  it  was  slow  to  be  accepted  by  the  public, 
until  checked  experimentally  wdth  the  greatest  precision,  and 
by  various  experimenters.  A  similar  saving  can  be  effected 
by  the  use  of  some  other  polyphase  combination  circuits,  but 
it  happens  that  the  three-phase  combination  is  the  one  least 
open  to  practical  objections. 


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188  ELECTRIC  TRANSMISSION  OF  POWER. 

In  actual  working  the  two-phase  system  is  nearly  always 
installed  with  a  complete  circuit  per  j)hase  as  regards  the  dis- 
tribution circuits,  unless  for  short  connections  to  apparatus; 
the  three-phase  system  is  used  with  the  star  or  mesh  combina- 
tion, except  for  occasional  special  work,  and  the  more  compli- 
cated polyphase  systems  are  practically  not  used  at  all,  save 
that  in  working  rotary  converters,  the  final  connection  is  some- 
times with  four,  six  or  even  more  phases  to  take  better  advan- 
tage of  the  armature  winding. 

In  speaking  of  the  voltage  of  an  alternating  circuit,  it  must 
be  borne  in  mind  that  we  do  not  mean  the  voltage  correspond- 
ing to  the  extreme  crest  of  the  E.  M.  F.  wave,  but  that  vol- 
tage which,  multiplied  by  the  current  in  a  non-inductive  circuit, 
equals  the  energy  in  that  circuit.  This  effective  working  vol- 
tage bears  no  fixed  relation  to  the  real  maximum  voltage,  since 


Fig.  M. 


their  ratio  evidently  varies  with  the  shape  of  the  E.  M.  F.  wave. 
For  a  sine  wave  the  ratio  is  1.414,  so  that  an  alternating  working 
pressure  of  1,000  volts  means  a  -maximum  voltage  of  1,414. 
As  may  be  judged  from  Fig.  91,  this  ratio  is  very  nearly  true 
for  the  best  modem  alternators. 

Save  in  rare  instances  the  work  of  power  transmission  is 
done  by  two-phase  or  three-phase  currents.  Abroad  some  pure 
single-phase  plants  are  in  operation  with  fairly  good  results, 
but  the  difficulty  of  getting  good  smgle-phase  motors  has  so  far 
rather  checked  development  along  this  line. 

In  this  comitr}',  a  decade  since,  the  so  called  "monocyclic" 
system,  now  obsolete,  was  introduced  in  a  few  plants  where  the 
motor  load  was  merely  incidental  to  lighting. 

In  this  system  there  was  a  main  armature  winding  to  which 
the  lighting  circuit  was  connected  as  in  ordinary  single-phase 
working,  while  a  subsidiary  armature  winding  furnished  mag- 


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TRANSMISSION  BY  ALTERNATING  CURRENTS.     189 

netizijig  current  for  the  motors.  The  general  arrangement 
of  the  armature  coils  is  shown  in  Fig.  94.  The  winding  in 
the  small  intermediate  slots  was  of  the  same  size  of  wire  as 
the  main  coil,  but  had  only  one-fourth  as  many  turns,  and 
consequently  one-quarter  the  main  E.  M.  F.  This  so-called 
"teaser"  E.  M.  F.  was  obviously  90^  in  phase  from  the  main 
E.  M.  F.  The  relation  of  the  two  E.  M.  F.'s  is  better  showTi 
in  Fig.  95,  where  A  B  C  is  the  main  E.  M.  F.  and  B  D  the  teaser . 
E.  M,  F.  The  generator  had  three  collecting  rings,  of  which 
the  middle  one  was  connected  to  D.  The  outer  rings  had  the 
full  E.  M.  F.  between  them,  while  between  D  and  C  the  E.  M.  F. 
was  the  geometrical  sum  oi  B  C  and  B  D,  approximately 
.56  of  the  main  E.  M.  F.  For  niotor  service  the  resultant 
E.  M.  F.^s  differing  in  phase  were  variously  combined,  usually 
into  approximately  three-phase  relation,  although  in  normal 


JT =^B  T: 

Fia.  95. 

rimning  all  the  currents  in  the  motor  remained  in  very  nearly 
the  same  phase.  The  object  of  this  system  was  to  obtain  for 
lighting  purposes  a  perfectly  simple  circuit,  the  voltage  of 
which  should  be  quite  undisturbed  by  actions  going  on  in  the 
subsidiary  motor  circuit,  which  object  was  attained  if  the 
generator  was  so  arranged  as  to  hold  its  voltage  closely  under 
inductive  load. 

A  similar  device  for  simplifying  the  operation  of  lighting 
circuits  is  a  three-phase  system  arranged  to  supply  the  entire 
lighting  service  from  two  of  it.s  lines,  as  A  and  B,  Fig.  92. 
The  other  two  connections  B  C  and  A  C  would  only  be  used 
for  motor  service,  and  if  desirable  the  coils  h  and  c  could  take 
up  very  little  space  on  the  armature.  Still  another  of  these 
heterophase  schemes  employs  regular  single-phase  alternators 
for  the  lighting  work,  and  a  small  adjunct  machine  in  phase  90° 
from  the  others,  and  connected  with  them  to  form  a  two-phase 


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190 


ELECTRIC  TRANSMISSION  OF  POWER, 


circuit  with  one  common  wire.  This  connection  is  used  for 
starting  ordinary''  two-phase  motors. 

In  general,  the  heterophase  systems  have  no  substantial 
advantage  over  the  ordinary  polyphase  systems,  and  are  rarely 
employed.  In  the  chapter  on  centres  of  distribution,  the 
working  properties  of  various  altematii>g  systems  wdll  be 
taken  up  in  more  detail. 

In  general  construction  and  arrangement  of  parts  all  alter- 
nators are  similar.     Those  specially  intended  for  power  trans- 


Fio.  %. 

mission  are  sometimes,  however,  modified  for  convenience  in 
obtaining  high  voltage  or  for  direct  coupling  to  water  wheels. 
The  vertical  shaft  arrangement  as  exemplified  in  the  original 
Niagara  machines  is  now  and  then  used  both  in  this  country  and 
abroad.  Machines  for  3,000  to  5,000  volts  and  upward  are 
best  constructed  with  stationary  armatures,  to  avoid  mechan- 
ical strains  on  the  high  voltage  insulation.  In  following  this 
design  the  armature  is  usually  exterior  to  the  field  magnets 
as  it  is  indeed  in  the  later  generators  of  the  great  Niagara  plant. 


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TRANSMISSION  BY  ALTERNATING  CURRENTS.     191 

It  is  very  doubtful  whether  the  fly-wheel  effect  gained  by 
revolving  an  exterior  magnet  compensates  for  the  great  inac- 
cessibility of  the  high  voltage  armature. 

A  characteristic  example  of  the  revolving  field  alternator  is 
shown  in  section  in  Fig.  96.*  It  is  a  large  polyphase  generator 
for  direct  connection  to  a  water-wheel,  and  the  cut  gives  a  good 
idea  of  its  mechanical  arrangement.  The  stationary  armature 
is  assembled  on  the  interior  of  a  supporting  circular  box  girder 
cast  in  upper  and  lower  halves.  In  the  fourth  quadrant  of  the 
cut  this  is  seen  in  section,  bearing  dovetailed  projections  for 
supporting  the  laminae  of  armature  iron.  These  are  curved 
segments  as  shown  in  the  third  quadrant,  twelve  segments  to 
the  entire  circle.  In  assembling  the  armature  each  layer 
breaks  joints  with  the  next,  and  when  the  whole  mass  of 
lamime  is  built  up  it  is  held  firmly  together  by  heavy  end  plates 
which  are  secured  by  bolts  passing  through  the  space  left 
between  the  laminse  and  the  supporting  girder.  This  stage  of 
the  construction  is  seen  in  the  second  quadrant.  Finally  after 
the  armature  coils  are  in  place  they  are  protected  by  a  seg- 
mental ventilated  shield  as  seen  in  the  first  quadrant.  The 
revolving  field  magnet  is  likewise  built  up  of  segmental  laminae 
dovetailed  to  supporting  castings,  which  are  in  turn  carried 
by  the  two  heavy  steel  plates  which,  bolted  to  the  hub,  form 
the  driving  spider.  As  in  most  such  constructions  the  pole 
tips  are  of  separate  laminae  dovetailed  or  interlocked  with  the 
laminje  of  the  polar  projections.  The  field  coils  are  held  in 
place  by  shoes  and  radial  bolts  to  relieve  the  pole  tips  of  the 
centrifugal  stress.  For  lower  speed  machines  the  poles  are 
often  solid  save  for  the  dovetailed  laminated  tips,  and  are 
simply  held  to  the  rim  of  the  field  spider  by  radial  bolts. 

The  construction  of  such  machines  is  very  various,  but  the 
main  point  is  that  the  high  voltage  windings  are  stationary, 
kept  well  clear  of  each  other,  and  singularly  accessible  so  that 
damaged  coils  are  very  easily  replaced.  Current  is  led  to  the 
field  by  two  small  slip  rings.  Even  for  low  voltage  machines 
this  construction  is  very  generally  preferred  by  reason  of  the 
greater  security  of  the  windings,  and  the  absence  of  the  large 
slip  rings  and  their  collecting  devices. 

•  See  Trans.  A.  I.  E.  E.  Feb.,  1004 


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192 


ELECTRIC   TRANSMISSIOX  OF  POWER. 


Still  another  form  of  alternator  in  which  the  armature, 
and  field  windings  as  well,  are  stationary,  is  found  in  the  ^* in- 
ductor" dynamo.  Of  this  the  most  familiar  types  are  those 
introduced  by  Mr.  Mordey  in  England  and  by  Mr.  Stanley  in 
this  country.  In  such  machines  the  magnetic  circuit  through 
the  armature  coils  established  by  fixed  field  coils  is  periodi- 
cally closed  and  opened  by  revolving  pole  pieces  which  them- 
selves carry  no  wire.     The  principle  is  illustrated  in  Fig.  97,  a 


Fio.  97. 

cross  section  of  the  Stanley  inductor  dynamo.  Here  the  cir- 
cular yoke  C  carries  two  rings  of  laminae  each  provided  with 
windings,  B,  arranged  much  as  in  the  alternator  just  described. 
Within  these  rings  revolve  two  sets  of  laminated  polar  pro- 
jections bonie  on  a  massive  spider  which  completes  the  mag- 
netic circuit.  The  stationary  field  winding  A  surrounds  the 
spider  as  a  whole  without  touching  it.  Evidently  all  the 
poles  at  one  end  of  the  spider  are  north  poles  and  those  at  the 
other  end  south  poles,  and  the  armature  coils  are  connected 


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Fig.  1. 


Fio.  2. 


PLATE  IV. 


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TRANSMISSION  BY  ALTERNATING  CURRENTS.     193 

accordingly.  As  a  rule  inductor  dynamos  are  not  economical 
of  material  owing  to  the  nature  of  the  magnetic  circuit,  and 
their  gain  in  security  and  simplicity  over  a  revolving  field 
alternator  of  the  ordinary  sort  is  hardly  enough  to  balance 
the  disadvantage,  so  that  they  are  now  less  used  than  formerly, 
although  in  themselves  excellent  machines. 

Plate  IV  shows  the  field  and  the  two  halves  of  the  armature 
of  a  modern  high  voltage  polyphase  generator  for  direct  con- 
nection to  the  prime  mover.  In  this  case  the  diameter  of  the 
armature  frame  is  so  great  that  it  has  been  found  desirable  to 
design  it  as  a  hollow  circular  truss  in  order  to  give  it  the  ne- 
cessary rigidity  against  distortion  by  its  own  weight  and  by 
inequality  of  magnetic  pull,  if  there  were  a  trifling  eccentricity 
due  to  wear  of  the  bearings.  In  some  of  the  early  machines 
of  large  diameter,  flexure  from  the  weight  alone  was  very 
troublesome.  Half  the  armature  coils  are  shown  in  place  and 
wedged  in,  and  a  coil  belonging  in  the  second  half  is  all  ready 
to  put  in  place.  Four  shapes  of  coils  are  necessary  to  com- 
plete this  winding,  but  they  can  be  kept  well  clear  of  each 
other  at  the  ends  and  are  easy  to  put  in  and  take  out,  so  that 
in  case  of  damage  a  coil  can  be  easily  replaced,  although  it 
may  sometimes  be  necessary  to  move  several  others  to  get  at 
the  damaged  one.  An  injured  coil,  however,  can  readily  be  put 
out  of  circuit. by  cutting  it  loose  at  the  ends,  msulating  them, 
and  connecting  the  adjacent  coils  of  the  same  phase  across  the 
dead  one.  A  generator  so  temporarily  repaired  in  a  few  min- 
utes can  be  run  imtil  opportunity  oiffers  for  permanent  repairs, 
and  can  even  be  worked  in  parallel  with  others  without  material 
difficulty. 

To  facilitate  repairs  the  armatures  of  large  revolving  pole 
machines  are  often  carried  on  a  sliding  bed,  so  that  they  can 
be  shifted  by  their  o^^ti  width  along  the  shaft,  exposing  the 
windings  of  both  armature  and  field. 

The  field  is  really  a  compact,  massive  fly-wheel  with  the 
poles  bolted  on  its  rim,  the  poles  surfaces  being  shaped  so  as  to 
give  as  nearly  as  may  be  a  sinusoidal  wave.  The  pole-pieces 
are  generally  laminated,  at  least  near  the  tips,  and  are  some- 
times provided  with  ventilating  spaces  like  those  in  the  ar- 
mature. 


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19-4 


ELECTRIC  TRANSMISSION  OF  POWER. 


The  advantage  of  revolving  field  generators  is  so  great  in 
point  of  easy  insulation  and  ready  collection  of  even  very  great 
currents,  that  this  type  of  machine  has  been  rapidly  displac- 
ing the  older  form  for  high  voltage  work,  and  indeed  for  large 
work  of  every  kind.  In  such  generators  voltages  of  10,000 
and  12,000  are  now  quite  common,  and  the  limit  has  not  been 
reached. 

Fig.  98  shows  the  efficiency  curve  of  one  of  the  huge  modern 
high  voltage  three-phasers.  It  is  from  a  5,000  KW,  11,000 
volt  directed  connected  generator  for  the  Interborough  Rapid 
Transit  Co.,  of  New  York  City,  and  while  it  does  not  show  the 


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FlO.  98. 

small  frictional  and  air  resistance  losses,  is  very  striking  as 
illustrating  first,  the  very  high  efficiency  reached  by  such 
machines,  and  second,  the  remarkably  uniform  efficiency  at 
varying  loads.  The  curve  passes  94  per  cent  at  a  little  below 
quarter  load,  reaches  a  full  load  efficiency  of  98  per  cent,  and 
rises  even  slightly  higher  on  a  25  per  cent  over  load.  The  regu- 
lation of  this  generator  is  also  excellent,  being  upon  non-induc- 
tive load  in  the  vicinity  of  5  per  cent.  It  is  built  with  a  revolv- 
ing 40  pole  field  32  feet  in  diameter  and  the  armature  winding 
is  distributed  in  four  slots  per  phase  per  pole,  each  slot  contain- 
ing three  bars. 

In  large  polyphase  generators  the  question  of  automatically 
regulating  the  voltage  in  response  to  changes  of  load  is  a  seri- 


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TRANSMISSION^  BY  ALTERNATING  CURRENTS.     195 

ous  one,  and  no  final  solution  of  it  has  as  yet  been  reached. 
It  is  not  economical  to  build  generators  with  so  small  inherent 
variation  of  voltage  as  is  in  itself  desirable.  In  small  poly- 
phase machines  compounding  has  been  accomplished  with  an 
arrangement  of  parts  similar  to  that  shown  in  Fig.  81,  the 
connections  being  so  modified  as  not  to  take  the  commutated 
current  from  a  single  phase.  This  is  troublesome  in  machines 
requiring  considerable  energy  for  the  field  excitation,  and  be- 
sides it  only  compounds  correctly  for  a  particular  value  of  the 
power  factor,  which  in  many  plants  is  constantly  changing. 
Several  modem  methods  of  compounding  direct  the  com- 


FlG.  99. 

pounding  at  the  exciter.  A  rotary  converter  is  used  as  ex- 
citer, and  the  voltage  at  its  commutator,  which  depends  on 
the  alternating  voltage  applied  at  the  slip-rings,  is  modified  in 
various  ways  in  response  to  changes  in  the  magnitude  and 
phase  of  the  working  currents  from  the  generator.  A  tj'-pical 
plan  of  this  kind,  successfully  applied  by  the  author  some  ten 
years  ago,  is  shown  diagrammatically  in  Fig.  99.  Here  the 
generator  fields  A'  A'  are  fed  from  the  commutator  end  of  a 
rotary  converter  F  F.  Current  from  the  main  collecting  rings 
a  is  led  to  the  collecting  rings  b  of  the  exciter  through  the  re- 
active coils  c  c  c  on  the  cores  M  M  il/,  which  are  also  wound 
with  series  turns  d  d  din  the  main  leads  of  the  generator.     At 


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196 


ELECTRIC  TRANSMISSION  OF  POWER. 


light  loads  the  voltage  at  b  is  cut  clown  by  the  reactance,  while 
as  the  main  current  increases  or  lags  the  series  turns  d  d  d  raise 
the  voltage  a  h,  and  hence  strengthen  the  generator  field.  By 
properly  proportioning  the  coils  c  c  c,  d  d  d,  and  theif  cores 
M  M  My  the  apparatus  can  be  made  to  regulate  the  voltage 
very  closely  for  all  loads  of  the  generator,  inductive  or  non- 
inductive,  or  even  may  over-compound  on  inductive  load  so  as 
to  compensate  for  the  change  in  the  inductance  of  the  system. 
Fig.  100  shows  the  working  of  this  device  when  arranged  to 
show  extreme  over-compounding  on  inductive  load.  The  gen- 
erator chosen  was  one  which  uncompounded  would  drop  its 
voltage  about  40  per  cent  on  a  heavy  inductive  load.     Curve 


160 
160 

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Fia.  100. 


A  shows  the  regulation  of  the  secondary  voltage  on  non-induc- 
tive load,  curs'^e  B  the  over-compoimding  produced  by  a  load  of 
induction  motors  running  light,  having  a  power  factor  of  not 
over  0.25. 

The  same  general  principle  has  been  lately  applied  in  several 
forms  with  very  promising  results.  An  interesting  modifica- 
tion is  the  compensated  field  alternator  recently  brought  out 
by  the  General  Electric  Company,  and  shown  in  Plate  V. 
Here  the  exciter  armature  is  on  the  shaft  of  the  main  machine, 
and  is  in  a  field  having  the  same  number  of  i)oles,  so  that  it 
revolves  synchronously  pole  for  pole  with  its  generator.  Ex- 
citer and  main  fields  are  fed  in  shimt  from  the  exciter  commu- 
tator, but  the  exciter  armatiure  also  receives  through  its  col- 


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-t.\ 


Fhj.  1. 


Fio.  2. 


PLATE   V. 


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TRANSMISSIOJSf  BY  ALTERNATING  CURRENTS.     197 

lector  rings  an  auxiliary  current  derived  from  series  trans- 
formers in  the  main  leads  of  the  generator.  This  device  holds 
the  voltage  with  beautiful  precision  under  ordinary  changes  of 
load  and  lag,  but  the  necessity  of  being  in  mechanical  S3mchron- 
ism  is  somewhat  embarrassing,  save  in  high  speed  machines. 
Another  very  pretty  method  of  regulation  by  compounding 
the  exciter,  is  that  due  to  Prof.  F.  G.  Baum*  and  shown  dia- 
grammatically  in  Fig.  101.  In  this  device  a  little  generator  of 
a  few  hundred  watts  capacity  is  mechanically  driven  in  syn- 
chronism with  the  main  generator  (?.  Its  fields  A  A'  are 
excited  by  a  few  turns  of  the  main  generator  current.     The 


Fig.  101. 

armature  B  has  a  very  simple  winding,  one  terminal  of  which 
goes  to  a  soUd  ring  connected  by  a  brush  with  the  lead  6,  the 
other  goes  to  a  pair  of  opposite  segments  about  90°  wide  and 
thence  to  the  lead  V .  When  the  fields  are  excited  from  the 
main  current  and  the  armature  is  turning  in  synchronism,  the 
machine  evidently  gives  a  partially  rectified  current,  more  or 
less  of  the  waves  of  one  polarity  being  bitten  off  short,  accord- 
ing to  the  position  of  the  segments  of  the  divided  ring.  The 
oscillograph  record  of  the  resulting  pulsatorj"-  current  is  shown 
in  Fig.  102.  Now  if  the  current  in  the  main  line  lags  the 
effect  is  precisely  the  same  as  if  the  segmental  ring  had  been 
turned  forward  a  little,  thus  increasing  the  amplitudes  of  the 
*  Trans.  A.  I.  E.  E.  May,  1902. 


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198  ELECTRIC  TRANSMISSION  OF  POWER. 

peaks,  and  hence  the  effective  E.  M.  F.  of  the  pulsatory  current. 
A  leading  current  produces  precisely  the  opposite  effect.  This 
automatically  varied  current  excites  the  field  of  a  second  little 
machine  C  which  compoimds  the  exciter  through  the  series 
coil  D,  The  rheostats  R  R  enable  the  compounding  to  be 
accurately  adjusted.  The  effect  of  this  apparatus  is  to  regu- 
late not  only  for  varying  current  but  for  variations  of  power 
factx)r,  in  a  very  satisfactory  manner.  It  may  also  be  applied 
to  synchronous  motors  and  rotary  converters. 

Along  such  lines  as  these,  good  results  are  certainly  attain- 
able, and  in  addition  there  are  several  automatic  devices  for 
working  a  rheostat  in  the  generator  field  so  as  to  hold  the  vol- 
tage constant,  irrespective  of  load  or  lag.  These  with  other 
regulating  apparatus  will  be  described  in  another  chapter. 


FlO.  102. 

As  a  matter  of  fact,  in  much  power  transmission  work  com- 
pound winding  is  not  necessary,  since  the  machines  hold  their 
voltage  closely  without  it  if  well  designed,  and  in  large  plants 
the  variations  of  load  are  usually  so  gradual  that  the  voltage  at 
the  end  of  the  transmission  line  can  be  easily  kept  constant 
by  hand  regulation.  Again,  in  many  transmission  plants  sev- 
eral lines  are  fed  by  one  generator,  so  that  no  compounding 
would  suit  all  the  lines;  and  whenever  a  substation  is  in- 
stalled, the  secondary  voltage  has  to  be  kept  constant  by 
special  regulation  in  any  event. 

TRANSFORMERS. 

The  alternating  current  transformer  is  merely  a  glorification, 
as  it  were,  of  the  fundamental  idea  showxi  in  Fig.  4,  page  12. 
The  loops  A  and  B  are  expanded  into  massive  coils  and  are 
given  a  very  perfect  magnetic  core  of  laminated  iron,  but  the 
principle  is  unchanged. 


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TRANSMISSIOJSf  BY  ALTERNATING  CURRENTS.     199 

In  Fig.  103,  ^  is  a  core  composed  of  soft  iron  plates  perhaps 
j^jj  inch  thick,  stamped  into  the  form  shown,  and  then  built 
up  together  like  the  leaves  of  a  book,  5  is  a  coil  of  insulated 
wire  wound  in  a  spiral  around  one  side  of  the  core,  and  C  is  a 
single  loop  of  heavy  insulated  copper  bar  around  the  other  side. 
Now  suppose  an  E.  M.  F.  is  suddenly  applied  to  the  terminals 
of  the  coil  By  the  loop  C  being  left  open.  Current  will  flow 
through  B  in  amount  determined  by  its  resistance  and  induc- 
tan,ce,  setting  up  a  magnetic  field  throughout  the  mass  of  A. 
If  the  current  is  an  alternating  one  an  alternating  magnetic 
field  "will  be  set  up  in  Ay  and  the  current  in  B  will  settle  down 
to  that  value  which  is  determined  by  the  resistance  and  induc- 


Fio.  103. 


tance  of  the  coil.  The  energy  represented  by  this  current  is 
spent  in  heating  the  coil  and  in  doing  work  by  the  reversal  of 
magnetism  in  the  core  A.  The  current  thus  engaged  lags 
behind  its  E.  M.  F.  as  in  other  cases  of  uiductive  circuit,  the 
power  factor  at  no  load  being  in  ordinary  cases  from  .6  to  .7. 
Now  close  the  loop  C  Current  opposing  the  current  in  B 
will  be  at  once  set  up.  The  magnetizing  effect  of  this  reverse 
current  opposes  the  magnetization  due  to  B,  and  hence  tends 
to  cut  down  the  inductance  imposed  on  By  which  is,  as  we  have 
already  seen,  determined  by  the  magnetic  induction  through 
its  core.  To  this  action  B  simultaneously  responds  wdth  an 
increased  current,  so  that  any  increase  of  the  current  in  C  and 
its  consequent  demagnetizing  action,  is  automatically  compen- 


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200  ELECTRIC  TRANSMISSION  OF  POWER. 

sated  by  an  increased  current  in  B,  The  increase  of  energy 
represented  by  this  compensates  for  the  energy  due  to  the 
current  in  C.  Energy  is  thus  virtually  transferred  from  the 
primary  circuit  B  to  the  secondary  circuit  C. 

Now  as  to  the  voltage  of  these  two  circuits.  The  energy 
in  the  two  circuits  is  evidently  equal  save  for  losses  in  the  iron 
and  copper,  which  amount  ordinarily  to  only  a  few  per  cent. 

For  any  given  magnetization  in  A  the  inductive  E.  M.  F. 
in  S  is  proportional  to  the  total  number  of  turns  in  the  coils; 
so  also  the  induced  E.  M.  F.  in  the  secondary  is  proportional 
to  the  number  of  turns  in  it.  That  is  for  a  certain  rate  of 
change  of  the  magnetic  induction  in  A,  the  induced  E.  M.  F. 
is  the  same  per  turn  throughout  A,  whether  that  E.  M.  F. 
appears  as  inductance  in  B  or  secondary  E.  M.  F.  in  C. 
Hence,  the  E.  M.  F.'s  across  the  terminals  of  the  primary  and 
secondary  coils  are  proportional  to  the  respective  numbers 
of  turns  in  those  coils.  But  the  energy  in  the  two  is  sub- 
stantially equal,  and  hence  the  currents  in  primary  and  secon- 
dary must  be  inversely  proportional  to  the  respective  E.M.  F.'s 
In  Fig.  103  are  shown  seven  primary  turns  and  one  secondary 
turn.  Therefore,  the  secondary  E.  M.  F.  is  one-seventh  the 
primary  E.  M.  F.,  and  the  primary  current  is  one-seventh  the 
secondary  current.  For  the  same  density  of  current  in  am- 
peres per  square  inch  the  secondary  turn  must  have  seven 
times  the  cross-section  of  the  primary  conductor.  By  simply 
changing  the  relative  number  of  primary  and  secondary  turns 
—  the  ratio  of  transformation  —  electrical  energy  at  any  vol- 
tage can  be  transformed  to  any  other  voltage  with  trifling  loss 
if  the  apparatus  be  properly  designed. 

The  losses  which  exist  arc  of  three  kinds.  First  is  the  loss 
due  to  the  resistance  of  the  copper.  This  at  light  loads  is 
very  trifling,  but  increases  with  the  square  of  the  load,  being 
numerically  equal  in  watts  to  C  -R,  as  in  all  cases  of  loss 
through  resistance. 

Second  comes  the  loss  through  hysteresis  —  virtually  mag- 
netic friction  —  produced  by  the  alternate  reversals  of  mag- 
netization in  the  iron  core.  This  is  nearly  constant  at  all 
loads  and  is  kept  as  low  as  possible  by  securing  the  best  pos- 
sible iron,  and  working  it  at  rather  low  magnetization,  since 


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TRANSMISSION  BY  ALTERNATING  CURRENTS.     201 

the  hysteretic  loss  increases  very  rapidly  as  the  iron  is  more  and 
more  strongly  magnetized. 

Finally  comes  the  loss  from  eddy  currents  in  the  core. 
This  is  due  to  the  fact  that  the  core  is  a  fairly  good  conductor, 
and  currents  are  induced  in  it  for  precisely  the  same  reason 
that  they  are  induced  in  the  secondary  winding.  These  eddy 
currents  are  largely  reduced  by  carefully  laminating  the  core 
across  the  natural  direction  of  flow  of  these  currents,  and 
insulating  the  laminae  with  sheets  of  tissue  paper  or  with 
varnish.  The  loss  from  eddy  currents  is,  generally  speaking, 
of  about  the  same  magnitude  as  the  hysteretic  loss,  and  in 
transformer  practice  the  two  are  usually  lumped  together  and 
denominated  core  loss. 

By  careful  construction  and  design  these  losses  can  be  kept 
very  small  compared  with  the  total  output.  The  following 
data  from  a  test  of  a  7,500  watt  transformer  designed  for  a 
frequency  of  15,000  to  16,000  alternations  per  minute,  about 
125  to  135  ^,  will  give  a  clear  idea  of  the  results  that  can  be 
reached  commercially  even  in  small  transformers. 

Output 7.5  KW 

Transformation  ratio 20 : 1 

Full  load  amperes  (primary) 3.6 

Full  load  amperes  (secondary) 72.0 

Resistance  (primary)  ohms 6.15 

Resistance  (secondary)  ohms      .        .        .        .        .        .        .  .012 

Total  C»  R  loss  (watts) 148. 

Total  core  loss  (watts) 78. 

Primary  cuiTent  (no  load) 063 

Power  factor  (no  load) .505 

Total  C  R  drop  (per  cent) 1.0 

The  efficiency  curve  of  this  transformer  at  various  loads  is 
given  in  Fig.  104.  The  interesting  feature  of  this  curve  is  the 
very  uniform  efficiency  from  half  load  to  full  load,  with  a  maxi- 
mum of  97.4  per  cent  at  three-quarters  load.  This  is  the 
result  of  a  relatively  very  small  core  loss.  Even  at  one-tenth 
the  normal  load  the  efficiency  is  still  good,  over  90  per  cent, 
although   the  curve  falls  more  rapidly  below  half  load. 

The  larger  transformers,  such  as  are  used  for  hea^'y  power 
transmission  work,  are  even  more  efficient  than  the  small  one 


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202 


ELECTRIC  TRANSMISSION  OF  POWER. 


here  described,  although  the  room  for  increase  is  now  very 
limited.  Within  the  last  few  years  the  improvement  in  com- 
mercial transformers  has  been  very  great.  In  practice  they 
are  seldom  so  simple  in  form  as  in  Fig.  103,  the  core  plates 


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OUTPUT  IN  K.W, 
Fia.  104. 

being  universally  built  up  of  several  pieces,  so  that  the  coils 
may  be  wound  in  forms  and  slipped  into  their  respective  places 
on  the  core.  One  of  the  forms  which  has  been  widely  used 
is  shown  removed  from  its  case  in  Fig.  105.  The  hollow 
rectangle  A  forms  the  main  part  of  the  core,  while  the  bridge 
piece,  B,  is  built  up  separately  as  the  core  of  the  coils,  together 


B 


Fig.  105. 


with  which  it  is  forced  into  the  position  shown.  The  secon- 
dary coil  immediately  surrounds  the  bridge,  and  outside  of  it 
is  the  primary  coil.  Both  coils  are  of  course  elaborately 
insulated.     Another  familiar  form  of  transformer  is  shown  in 


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TRANSMISSION  BY  ALTERNATING  CURRENTS,     203 

Figs.  106  and  107.  Here  the  core  is  built  up  of  straight  rec- 
tangular slips  of  iron  into  a  hollow  rectangle  upon  the  longer 
sides  of  which  the  coils  are  fitted,  as  in  Fig.  106,  separated 
by  heavy  sheet  insulation  in  the  manner  shown.  The  whole 
assembled  core  and  coils  are  shown  in  longitudinal  section  in 
Fig.  107.  This  form  of  construction  gives  the  coils  a  large 
available  cooling  surface  and  simplifies  their  insulation  some- 


FlO.  106. 

what,  although  magnetically  the  arrangement  of  Fig.  105  is 
to  be  preferred. 

As  transformers  are  usually  inclosed  in  tight  iron  boxes  to 
protect  them  from  the  weather,  the  heat  generated  in  the  coils 
and  core  has  a  rather  poor  chance  to  escape,  and  the  tempera- 
ture may  therefore  rise  higher  than  is  safe  for  the  insulation. 
It  is  usual  to  take  special  precautions  to  prevent  this  over- 
heating.    One  of  the  commonest  and  best  devices  for  this 


Fig.  107. 

purpose  is  the  subdivision  of  the  core  into  bunches  of  laminse 
separated  by  air  spaces. 

This  arrangement  is  well  shown  in  Fig.  108,  in  which  the 
core  is  provided  with  a  dozen  of  these  ventilating  spaces. 
The  arrangement  of  the  coils  is  somewhat  like  that  of  Fig.  105. 
As  an  additional  precaution  against  overheating,  the  trans- 


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204  ELECTRIC  TRANSMISSION  OF  POWER. 

former  case  is  often  filled  with  heavy  mineral  oil  after  the 
core  is  in  place.  This  both  provides  additional  insulation, 
and  facilitates  the  transfer  of  heat  from  the  core  and  coils  to 
the  iron  case,  whence  it  is  radiated  to  the  surroimding  air.  In 
very  large  transformers  the  primary  and  secondary  windings 
are  often  built  up  of  thin  flat  sections  assembled  with  spaces 
between  them. ' 

For  huge  transformers  such  as  are  used  for  substation 
work,  means  are  generally  provided  for  artificial  cooling.  Two 
methods  are  at  present  in  use  for  this  purpose.  One  is  the 
use  of  a  blast  of  air  from  a  small  blower  streaming  through 


Fio.  108. 

the  interstices  provided  in  core  and  coils,  and  rapidly  carrying 
away  the  heat  generated.  The  other  is  appUed  to  oil-filled 
transformers,  and  consists  in  cooHng  the  oil  by  a  worm  in 
the  transformer  case  through  which  cold  water  is  allowed 
to  flow,  or  with  a  small  pump  circulating  the  oil  itself  slowly 
through  a  worm  cooled  by  water.  Either  plan  is  very  effective, 
and  both  are  extensively  used. 

With  properly  designed  tranformers  there  is  no  difficulty  in 
dealing  with  any  voltage  now  in  use,  without  the  device  of 
connecting  transformers  in  series,  which  was  formerly  often 
employed  for  high  voltage.  Plate  VI  shows  a  type  of  the 
latest  transformer  practice  in  an  oiled-cooled  900  KW  West- 
inghouse  transformer.  It  is  designed  for  use  at  25  -^  to  give 
60,000  volts  upon  the  transmission  lines.     Its  splendid  efficiency 


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PLATE  VI. 


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TRANSMISSION  BY  ALTERNATING  CURRENTS.     206 

curve,  which  speaks  for  itself,  is  shown  iji  Fig.  109.  If  trans- 
formers are  of  similar  size  and  design,  they  can  be  run  in 
parallel  with  the  utmost  facility,  and  may  very  often  be  thus 
"banked"  most  advantageously,  as  with  such  comiection  it 
is  easy  to  proportion  the  number  of  transformers  in  use  to  the 
load,  so  that  they  can  be  worked  nearly  at  full  load,  and  con- 
sequently at  their  best  efficiency. 

In  general  the  larger  the  transformer  the  higher  its  effi- 
ciency, though  the  improvement  is  very  slow  after  the  out- 
put reaches  25  KW  or  thereabouts.     The  curve  of  Fig.  110 


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PROPORTION  OF  FULL  LOAD 

Fio.  100. 

shows  the  change  in  half -load  efficiency  with  the  size  of  trans- 
former as  found  in  ordinary  American  practice. 

The  data  here  given  relate  to  transformers  of  the  kind 
employed  for  power  transmission  work,  as  now  produced  by 
the  best  makers.  The  sizes  above  50  KW  are  frequently 
artificially  cooled.  The  frequency  is  taken  at  60  '^  to  70  ^, 
and  the  figures  do  not  apply  to  transformers  originally 
designed  for  higher  frequencies.  At  lower  frequencies  the 
efficiencies  are  Ukel)'^  to  be  a  fraction  of  a  per  cent  lower,  but 
at  any  frequency  within  the  range  of  ordinarj"  working  a  first- 
class  transformer  of  50  KW  capacity  or  upward  can  be 
depended  on  for  a  full  load  efficiency  of  just  about  98  per 


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206 


ELECTRIC  TRANSMISSION  OF  POWER, 


cent,  and  a  half  load  efficiency  about  one  per  cent  lower. 
With  care  m  planning  a  substation  equipped  with  these  large 
transformers,  the  loss  under  normal  conditions  of  working 
should  not  exceed  2^  per  cent. 

For  polyphase  work  it  is  the  almost  universal  custom  in  this 
country  to  employ  simply  groups  of  ordinary  standard  trans- 
formers. Abroad,  composite  transformers,  transforming  two 
or  more  phases  in  a  single  structure,  are  often  used.  The 
intent  of  this  arrangement  is  to  utilize  more  fully  the  iron 
core  by  making  it  common  to  the  several  phase  windings. 
Three  laminated  cores,  with  the  laminae  running  vertically,  are 


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K.W.  OUTPUT 

no.  110. 

united  at  the  ends  by  laminated  yokes.  Each  core  receives 
the  primary  and  secondary  windings  belonging  to  a  single 
phase,  while  the  iron  belongs  to  the  three  in  common.  The 
arrangement  is  akin  to  the  mesh  connection  of  three-phase 
circuits. 

It  is  a  question  whether  the  common  use  of  the  core  iron 
is  a  sufficient  offset  to  the  loss  incurred  in  operative  flexibihty. 
Separate  transformers  for  each  phase  can  be  readily  shifted 
about  or  reconnected  in  case  of  accident,  while  if  anything 
happens  to  a  polyphase  transformer  it  is  likely  to  put  out  of 
action  a  considerably  greater  capacity  than  in  the  other  case. 
Nevertheless,  three-phase  transformers  are  considerably  used 
abroad  and  very  recently  they  have  come  into  current  prac- 


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TRANSMISSION  BY  ALTERNATING  CURRENTS.     207 

tice  here.  Fig.  Ill  shows,  removed  from  its  oil-filled  ease,  a 
three-phase  transformer  of  about  50  KW  capacity.  The 
arrangement  of  the  cores  is  akin  to  that  of  Fig.  107,  but  with 
three  wound  cores  instead  of  two.  Similar  transformers  are 
now  being  made  of  several  thousand  kilowatts  capacity,  but 
whether  they  will  have  a  permanent  place  in  the  art  remains 
to  be  seen.  They  are  at  present  emphatically  special,  and  it 
is  somewhat  dubious  whether  they  present  sufficient  advan- 
tages to  compensate  for  the  extra  capacity  jeopardized  in  case 
of  trouble. 


Fio.  u 

Several  arrangements  of  transformers  are  employed  in  poly- 
phase working  corresponding  to  the  various  arrangements  of 
polyphase  circuits.  For  example,  in  two-phase  systems  the 
transformers  are  generally  connected  as  shown  in  Fig.  112. 
This  is  simply  one  transformer  per  phase  connected  in  the  ordi- 
nary manner.  The  two  phases  are  kept  distinct  both  as  regards 
primary  and  secondary  sides  of  the  circuit.  Fig.  113  shows 
the  composite  circuit  method  of  connection.  Both  primary 
and  secondary  circuits  have  one  wire  common  to  both  phases. 
In  this  case  there  is  between  the  outside  wires  of  the  system 
a  higher  voltage  than  exists  between  either  outside  wire  and 
the  common  wire.    This  voltage  is  of  course  the  geometrical 


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208 


ELECTRIC  TRANSMISSION  OF  POWER. 


sum  of  two  separate  phase-voltages.  As  these  are  90® 
apart  the  resultant  voltage  is  V2  times  either  component. 
Not  infrequently  the  primary  arrangement  of  Fig.  112  is  com- 
bined with  the  secondary  circuit  of  Fig.  113.  This  is  the  ordi- 
nary connection  of  two-phase  motors,  which  are  often  built  for 
this  three-wire  circuit.  As  a  rule  all  lighting  connections  and 
all  long  circuits  of  any  kind  are  made  as  shown  in  Fig.  112. 

Transformers  for  three-phase  circuits,  are,  like  the  circuits 
themselves,  very  seldom  worked  with  the  phases  separated, 
but  in  nearly  every  case  are  combined  in  the  star  or  mesh 


Fig.  112. 

connection.  The  former  is  useful  in  dealing  with  very  high 
voltages,  since  the  individual  transformers  do  not  have  to  carry 
the  full  voltage  between  lines.  Fig.  114  shows  a  diagram  of 
the  star  connection  and  Fig.  115  the  corresponding  mesh.  In 
each  a,  6,  c,  are  the  primary  leads,  and  A,  B^C  the  correspond- 
ing secondary  leads.  Of  the  two  connections  the  mesh  is 
rather  the  more  in  use  except  for  high  voltage  work,  and  for 
secondary  distribution  with  a  connection  to  the  common 
junction  of  the  transformer  system,  which  connection  has  for 
certain  purposes  very  great  advantages. 
Whether  the  star  or  the  mesh  connection  is  employed,  one 


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TRANSMISSION  BY  ALTERNATING  CURRENTS.     209 


transformer  per  phase  is  required,  and  this  condition  is  some- 
times inconvenient  as  rendering  necessary  the  use  of  three 
small  transformers  where  a  two-phase  system  would  need  but 
two.    To  obviate  this  difficulty,    what    may    be    called    the 


A/WW\ 

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FlO.  113. 


"resultant  mesh"  connection  is  extensively  used,  particularly 
for  motors.    The  principles  on  which  this  is  based  have  already 
been  set  forth. 
Briefly,  if  one  takes  the  geometrical  sum  of  two  E.  M.  F.'s 


b  <h 


not  in  phase  with  each  other,  the  resultant  will  be  less  than  the 
arithmetical  sum  of  the  components,  and  not  in  phase  with 
either.  From  the  examples  of  geometrical  summation  already 
discussed,  it  is  evident  that  by  varying  the  magnitudes  of  the 
components  and  the  angle  between  them,  i.e.,  their  phase 
difference,  the  resultant  may  have  any  desired  value  and  any 
direction  with  reference  to  either  component. 


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210 


ELECTRIC  TRANSMISSION  OF  POWER. 


The  "resultant  mesh"  three-phase  connection  is  shown 
in  Fig.  116.  It  is  composed  of  two  transformers  instead  of 
three  as  in  Fig.  115,  the  E.  M.  F.  between  the  points  A  and  C 
being  the  resultant  derived  from  the  two  existing  secondaries. 
Each  of  these  secondaries  contributes  its  part  of  the  output 
in  the  resultant  phase,  and  the  secondary  circuit  behaves 
substantially  as  if  it  were  derived  from  the  ordinary  mesh 
connection.  This  arrangement  is  very  convenient  in  motor 
work,  since  it  is  very  simple  and  allows  the  use  of  two  trans- 
formers when  desirable  for  the  required  output.  Sometimes 
a  motor  is  of  a  size  that  is  fitted  better  by  three  standard 
transformers  than  by  two,  or  the  reverse,  and  with  the  choice 


FIO.  115. 


of  the  two  mesh  connections  it  is  often  possible  to  avoid  some 
extra  expense  or  to  utilize  transformers  that  are  on  hand. 

A  very  beautiful  application  of  this  principle  of  resultant 
E.  M.  F.  is  the  change  of  a  two-phase  system  into  a  three- 
phase,  or  vice  versa.  The  method  of  doing  this  is  shown  in 
Fig.  117.  Suppose  we  have  two  equal  E.  M.  F.'s  90°  apart,  as 
in  the  ordinary  two-phase  system,  as  the  primary  circuit.  The 
secondary  E.  M.  F.'s  will  still  be  90®  apart,  but  can  be  of  any 
magnitude  we  please.  Let  one  of  these  secondaries  A  C  give 
say  100  volts,  and  tap  it  in  the  middle  so  that  the  halves,  A  D 
and  D  C  will  each  be  50  volts;  now  wind  the  other  secondary, 
B  Dy  for  50  V3  volts,  and  connect  one  end  of  it  to  the  middle 
point  of  the  first  secondary.    Taking  now  the  geometrical 


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TRANSAfIS;SlON  BY  ALTERNATINC  CURRENTS.     211 

sums  of  B  D  with  the  two  halves  of  A  C,  the  resultants  are 
equal  to  each  other  and  to  A  C,  and  leads  connected  to  i4,  B, 
and  C  will  give  three  equal  E.  M.  F.s  120®  apart,  forming  a 
three-phase  mesh  with  two  resultant  E.  M.  F.'s  instead  of  one 


Pig.  116. 


as  in  Fig.  116.  The  actual  connection  of  a  1,000  volt  two-phase 
system  to  form  a  100  volt  three-phase  secondary  system  is 
shown  in  Fig.  118  .  Reversing  the  operation  by  supplying 
three-phase  current  to  the  three-phase  side  of  the  system 
gives  a  resultant  two-phase  circuit. 

This  change-over  process  is  valuable  in  that  it  allows  a 


three-phase  transmission  circuit  to  be  used  for  the  sa^'ing  in 
copper  characteristic  of  it,  in  connection  with  two-phase 
generating  and  distributing  plants,  and  permits  two-phase  and 
three-phase  apparatus  to  be  used  interchangeably  on  the  same 


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212 


ELECTRIC  TRANSMISSION  OF  POWER, 


circuit,  wh\ch  is  sometimes  advantageous.  A  somewhat 
analogous  arrangement  permits  the  transformation  of  a 
monocyclic  primary  circuit  into  a  three-phase  or  two-phase 
secondary  form,  as  may  be  convenient,  and  in  fact  any  sys- 
tem with  two  or  more  phases  may  be  transformed  into  any 
other  similar  system  in  the  general  manner  described. 

It  is  worth  noting  that  the  three-phase-two-phase  trans- 
formation sho\\Ti  in  Fig.  118  can  in  an  emergency  be  very 
readily  made' without  special  transformers  if.  one  has  avail- 
able transformers  of  ratios  9:  1  and  10:  1,  respectively,  both 


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Fio.  118. 

these  being  obtainable  commercially.  For  the  latter  tapped 
from  the  middle  of  the  secondary,  as  is  common  for  three- 
wire  work,  gives  the  left-hand  half  of  Fig.  118,  while  the  9:  1 
transformer  is  sufficiently  near  the  required  ratio  to  give  the 
rest  of  the  combination.  Such  an  extemporized  arrangement 
is  very  serviceable  in  operating  three-phase  induction  motors 
from  two-phase  mains  or  vice  versa,  and  can  be  put  together 
very  easily.  In  default  of  this  it  is  easy  enough  in  using 
standard  transformers  of  makes  in  which  the  secondary  wind- 
ings are  fairly  accessible,  to  tap  the  secondary  winding  so  as 
to  leave  about  12  per  cent  of  it  dead-ended,  and  this  forms 
the  supplementary  transformer  required. 

The  electrician  will  do  well  to  familiarize  himself  with  the 
handling  of  transformers  in  all  sorts  of  connections,  for  in  a 


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TRANSMISSION  BY  ALTERNATING  CURRENTS.     213 

sudden  emergency  a  little  deftness  in  this  respect  will  often 
extricate  him  from  an  uncomfortable  comer.  For  instance, 
one  can  connect  transformers  backwards  to  get  high  voltage 
for  testing,  or  with  the  usual  three-wire  secondaries  trans- 
form twice  and  reach  half  the  primary  voltage,  or  put  several 
secondaries  in  series,  with  the  corresponding  primaries  in  mul- 
tiple, or  do  many  other  things  occasionally  useful.  The  chief 
things  to  be  borne  in  mind  are  that  the  normal  currents  in 
primaries  and  secondaries  must  not  be  exceeded,  that  the 
polarities  must  be  kept  straight  and  great  care  must  be  exer- 
cised not  inadvertently  to  get  any  coils  on  short  circuit. 


To  Line- 


c^ 


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FlQ.  119. 


One  of  the  most  useful  temporary  expedients  is  boosting  the 
primary  voltage  by  means  of  a  standard  transformer  to  meet 
excessive  drop  in  a  long  feeder.  The  process  is  exceedingly 
simple,  being  merely  the  connection  of  the  secondary  in  series 
with  the  line  to  be  boosted,  while  the  primary  is  put  across  the 
mains  as  usual.  The  result  is  that  the  feeder  voltage  is  raised 
by  nearly  the  amount  of  the  secondary  voltage.  Fig.  119 
shows  a  convenient  way  of  arranging  the  connections,  in  which 
one  of  the  primary  lines  is  so  connected  to  a  double  throw 
single-pole  switch  that  while  boosting  goes  on  with  the  switch 
in  the  position  shown,  on  throwing  the  switch  to  the  reverse 
position  the  booster  is  cut  out  and  the  line  receives  its  current 
as  usual.    It  must  be  remembered  in  such  boosting  that  the 


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214  ELECTRIC  TRANSMISSION  OF  POWER. 

strain  on  the  transformer  insulation  is  more  severe  than  usual, 
and  in  particular  that  the  strain  between  secondary  coils  and 
core  is  the  full  primary  voltage,  for  which  provision  is  seldom 
made  in  insulating  secondaries  from  cores.  Hence,  in  rigging 
a  booster  transformer  one  of  the  oil-insulated  type  should  be 
chosen,  and  it  should  be  very  carefully  insulated  from  the 
ground.  For  the  same  reason  the  boosting  transformer 
should  be  of  ample  capacity,  so  that  it  will  not  be  likely  to 
overheat,  and  should  in  general  be  treated  rather  gingerly,  like 
any  other  piece  of  apparatus  subject  to  unusual  conditions. 
Nevertheless,  it  is  capable  of  most  effective  service  if  properly 
operated. 

All  these  systems  which  involve  resultant  E.  M.  F.'s  are 
open  to  certain  practical  objections  which  may  or  may  not  be 
important  according  to  circumstances. 

In  the  first  place,  the  resultant  E.  M.  F.  is  less  than  the 
sum  of  the  E.  M.  F/s  for  which  the  transformers  in  the  com- 
ponent circuits  are  wound.  For  instance,  in  Figs.  116  and  118, 
100  resultant  volts  are  derived  from  transformers  aggregating 
respectively  200  and  186.7  volts,  through  the  secondaries  of 
which  the  resultant  current  has  to  flow.  In  the  former  case 
one-third  and  in  the  latter  case  two-thirds  of  the  total  current 
is  thus  derived  at  a  disadvantage,  using  up  more  transformer 
capacity  for  a  given  amount  of  energy  than  if  the  transformers 
were  used  in  the  normal  manner.  On  a  small  scale  the  dis- 
advantage is  seldom  felt,  but  in  heavy  transmission  work  with 
large  transformers  it  may  be  quite  serious. 

Second,  the  disturbance  of  any  one  component  voltage  from 
drop  or  inductance,  or  any  shifting  of  phase  between  the  com- 
ponents from  unequal  lag,  disturbs  all  the  resultant  E.  M.  F.'s 
This,  again,  may  or  may  not  be  of  importance,  but  it  must 
always  be  borne  in  mind,  as  ia  every  case  of  combined  phases. 

It  is  possible  by  combinations  of  transformer  similar  to 
those  described  to  obtain  at  some  sacrifice  in  transformer 
capacity  a  single-phase  resultant  E.  M.  F.  from  polyphase 
components,  or  to  split  up  a  single-phase  current,  by  the  aid  of 
inductance  and  capacity,  into  polyphase  currents.  Neither 
process  is  employed  much  commercially,  since  both  encounter 
in  aggravated  form  the  difficulties  common  to  resultant  phase 


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TRANSMISSION  BY  ALTERNATING  CURRENTS.     215 

working  mentioned  above,  and  others  due  to  the  special  form 
of  the  combinations  attemptcnl.  Combining  polyphase  cur- 
rents for  a  single-phase  resultant  is  a  process  that  would  be 
very  seldom  useful,  but  the  reverse  process  if  it  were  success- 
fully carried  out  might  l>e  of  very  great  importance  in  certain 
distribution  problems,  and  especially  in  electric  railway  prac- 
tice, although  in  working  on  the  large  scale  that  offers  the  best 
field  for  alternating  motors  the  disadvantage  of  two  trolleys  is 
at  a  minimum.  One  very  ingenious  method  of  splitting  an  al- 
ternating current  into  three-phase  components  is  the  following, 
due  to  Mr.  C.  S.  Bradley,  one  of  the  pioneers  in  polyphase 


VlG.  120. 

work.  His  process  is  essentially  twofold,  first  splitting  the 
original  current  into  a  pair  of  components  in  quadrature 
and  then  combining  these  somewhat  as  in  Fig.  118.  The  appa- 
ratus is  shown  in  diagram  in  Fig.  120.  Here  A  is  the  gen- 
erator, B  the  simple  primary  of  one  transformer  elenient,  D  a 
condenser,  n  and  I  the  sections  of  the  compound  transformer 
primary,  and  g,  A,  i,  /,  k  the  secondary  transformer  sections. 
The  condenser  D  is  so  proportioned  that  acting  in  conjunc- 
tion with  the  compound  primary  n  I  the  original  current 
is  split  into  two  components  in  quadrature,  in  B  and  n  I  respec- 
tively. Then  the  secondaries  are  so  intercoimected  as  to 
produce  three-phase  resultant  currents  which  are  fed  to  the 


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216  ELECTRIC  TRANSMISSION  OF  POWER. 

motor  M.  The  coil  i  gives  one  phase,  the  resultant  of  g  and  k 
another,  and  the  resultant  of  h  and  /  the  third.  The  combi- 
nation of  these  resultants  gives  a  more  uniform  and  stable 
phase  relation  under  varying  loads  than  would  be  obtained 
from  two-phase  secondaries  fed  by  B  and  n  I  respectively. 
The  condenser  is  necessary  in  getting  a  correct  two-phase  re- 
lation in  the  primaries  to  start  with,  and  even  so  the  E.  M.  F.'s 
will  not  stay  in  quadrature  imder  a  varying  load  on  the  sec- 
ondaries unless  the  condenser  capacity  be  varied,  but  the  re- 
combination in  the  secondaries  partially  obviates  this  difficulty. 
A  device  brought  out  abroad  by  M.  Korda  for  a  similar  purpose 
omits  the  condenser  and  splits  the  monophase  current  into  two 
components  60°  apart  by  variation  of  inductance  alone,  and 
these  are  utilized  to  give  three-phase  resultants.  The  phase 
relations  thus  obtained  are,  however,  unstable,  as  must  always 
be  the  case  in  phase  splitting  by  inductance  alone.  For 
the  energy  supplied  by  a  monophase  current  is  essentially 
discontinuous,  while  the  energy  of  a  polyphase  circuit  has  no 
periodic  zero  values,  so  that  in  passing  from  one  to  the  other 
there  should  be  storage  of  energy  during  part  of  each  cycle  such 
as  is  obtained  by  the  condenser  of  Fig.  120. 


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CHAPTER  VI. 

ALTERNATING   CURRENT   MOTORS. 

The  principles  of  the  synchronous  alternating  motor  are  a 
snare  for  the  unwary  student  of  alternating  current  working, 
since  they  involve,  when  discussed  in  the  usual  way,  rather 
complicated  mathematical  considerations.  And  the  worst  of 
it  is  that  the  generalized  treatment  of  the  subject  often  causes 
one  to  lose  sight  of  the  fundamental  ideas  that  are  at  the  root  of 
alternating  and  continuous  current  motors  alike.  The  sub- 
ject is  at  best  not  very  simple,  and  unless  we  are  prepared  to 
attack  the  general  theory  with  all  its  many  considerations,  it  is- 
desirable  not  to  cut  loose  from  the  common  basis  of  all  motor 
work. 

Recurring  to  the  rudimentary  facts  set  forth  in  Chapter  T, 
we  see  that  an  electric  motor  consists  essentially  of  two  work- 
ing parts  —  a  magnetic  field  and  a  movable  wire  carrying  an 
electric  current.  The  motive  power  —  torque  —  is  due  to  the 
reaction  between  the  magnetic  stresses  set  up  by  the  current 
and  those  due  to  the  field.  The  refinements  of  motor  design  are 
concerned  with  the  efficient  production  of  these  two  sets  of 
stresses  and  their  coordination  in  such  wise  that  their  reac- 
tion shall  produce  a  powerful  torque  in  a  uniform  direction. 

In  continuous  current  motors,  for  example,  the  field  mag- 
nets are  energized  by  a  part  or  the  whole  of  the  working- 
current,  and  this  current  is  passed,  before  entering  the  arma- 
ture, through  a  commutator  like  that  of  the  generator,  so 
that  in  the  armature  the  direction  of  the  currents  through  the 
working  conductors  shall  be  reversed  at  the  proper  time,  so  as 
to  react  in  a  uniform  direction  with  field  poles  which  are  con- 
secutively of  opposite  polarity.  Were  it  not  for  the  commu- 
tator the  armature  would,  on  turning  on  the  current,  stick  fast 
in  one  position,  as  may  happen  when  there  is  a  defect  in  the 
winding. 

Now,  since  the  function  of  the  commutator  in  the  generator 

m 


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218  ELECTRIC  TRANSMISSION  OF  POWER, 

is  to  change  a  current  normally  alternating,  so  that  it  shall 
flow  continuously  in  one  direction,  and  since  the  object  of  the 
commutator  in  the  motor  is  periodically  to  reverse  this  current 
in  the  armature  coils,  thus  getting  back  to  the  original  current 
again,  one  naturally  asks  the  reason  for  going  to  all  this  trouble. 
Why  not  let  the  generator  armature  do  the  reversing  instead 
of  providing  two  commutators  —  the  second  to  undo  the  work 
of  the  first? 

The  reason  is  not  far  to  seek.  In  a  generator  running  at 
uniform  speed  the  reversals  of  current  take  place  at  certain 
fixed  times  —  whenever  an  armature  coil  passes  from  pole  to 
pole,  quite  irrespective  of  the  needs  of  the  motor.  The  com- 
mutator on  the  other  hand  reverses  the  current  in  the  motor 
armature  coils  in  certain  fixed  positions  with  respect  to  the 
field  poles  so  as  to  pn>duce  a  continuous  pull,  irrespective  of 
what  the  generator  is  doing. 

If  we  abolish  the  commutators  the  motor  will  nm  properly 
only  when  the  alternating  impulses  received  from  the  gen- 
erator catch  the  armature  coils  systematically  in  the  same 
positions  in  which  reversal  would  be  accomplished  by  the 
coitimutator.  Hence  for  a  fixed  speed  of  the  generator  the  im- 
pulses will  be  properly  timed  only  when  the  motor  armature 
is  turning  at  such  a  speed  that  each  coil  passes  its  proper 
reversal  point  simultaneously  with  each  reversal  of  the  genera- 
tor current.  If  generator  and  motor  have  the  same  number 
of  poles,  this  condition  will  be  fulfilled  only  when  they  are 
running  at  exactly  the  same  number  of  revolutions  per  minute. 
In  any  case  they  must  run  synchronously  pole  for  pole,  so  that 
if  the  motor  has  twice  as  many  poles  as  the  generator,  it  will 
be  in  synchronism  at  half  the  speed  in  revolutions  per  minute, 
and  so  on. 

If  we  try  to  dispense  with  the  commutators  when  starting 
the  motor  from  rest,  the  action  will  obviously  be  as  follows: 
The  first  impulse  from  the  generator  might  be  in  either  direc- 
tion, according  to  the  moment  at  wliich  the  switch  was  thrown. 
The  reaction  between  this  current  in  the  armature  coils  and 
the  field  poles  might  tend  to  pull  the  armature  in  either  direc- 
tion, but  long  before  the  torque  could  overcome  the  inertia 
of  the  armature  a  reverse  impulse  would  come  from  the  gen- 


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ALTERNATING  CURRENT  MOTORS.  219 

erator  and  undo  the  work  of  the  first.     Consequently  the  motor 
would  fail  to  start  at  all. 

If  the  impulses  from  the  generator  came  very  slowly  indeed, 
so  that  the  first  could  give  the  armature  a  start  before  the 
second  came,  the  armature  would  stand  a  chance  of  getting 
somewhere  near  its  proper  reversal  point  before  the  arrival  of 
the  reverse  current,  and  thus  might  get  a  helping  pull  that 
would  improve  matters  at  the  next  reversal,  but  the  direction 
of  the  first  impulse  would  be  quite  fortuitous.  Starting  the 
armature  in  either  direction  before  the  current  is  thrown  on 
gives  it  a  better  chance  to  go  ahead  if  the  first  impulses  in  the 
wrong  direction  are  not  strong  enough  to  stop  it  altogether. 


00000  '^tt%r    00000 

Fio.  121. 

We  see,  then,  that  an  alternating  current  derived  directly 
from  the  generator  does  not  give  reversals  in  the  motor  coils 
that  are  equivalent  to  the  action  of  a  commutator,  save  at 
synchronous  speed.  Except  at  this  speed  the  current  from 
the  generator  does  not  reverse  in  the  motor  armature  coils 
when  the  latter  are  in  the  proper  position. 

Fig.  121  will  give  a  clear  idea  of  the  conditions  of  affairs  in 
the  field  and  armature  conductors  of  a  continuous  current 
motor.  Here  S  and  A^  are  the  poles,  and  +  and  —  mark  the 
positions  of  the  positive  and  negative  brushes  with  reference 
to  the  armature  winding.  The  solid  black  conductors  carry 
current  flowing  down  into  the  plane  of  the  paper.  The  white 
conductors  carry  current  upward.  The  armature  turns  in 
the  direction  of  the  arrow,  and  as  each  conductor  passes  under 
the  brush  the  current  in  it  is  reversed.    This  distribution  of 


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220 


ELECTRIC  TRANSMISSION  OF  POWER. 


current  is  necessary  to  the  proper  operation  of  the  motor,  and 
if  the  brushes  are  moved  the  motor  will  run  more  and  more 
weakly,  and  then  stop  and  begin  to  rim  in  the  opposite  direc- 
tion, imtil  when  the  brushes  have  moved  180°  the  motor  will 
be  running  at  full  power  in  the  reverse  direction.  This  final 
position  means  that  the  currents  in  the  two  halves  of  the 
armature  have  exchanged  directions,  so  that  the  conductors 
originally  attracted  toward  N  and  repelled  from  S,  are  now 
repelled  from  N  and  attracted  toward  S,  If  alternating 
current  from  the  generator  is  led  into  the  windings,  the  dis- 
tribution of  current  showTi  in  Fig.  121  must  be  preserved,  and 
since  in  abolishing  the  commutator  the  alternating  current 
leads  are  permanently  connected  to  two  opposite  armature 
coils  through  slip  rings,  the  distribution  of  Fig.  121  can  only 


Fro.  122. 


be  preserved  when  these  leads  change  places  by  making  a  half 
revolution  every  time  the  current  reverses  its  direction.  Other- 
wise the  distribution  of  currents  will  be  changed,  and  the 
motor  will  fail  to  operate,  since  each  reversal  of  current  will 
catch  the  armature  in  a  wrong  position,  and  may  tend  to 
turn  it  in  the  wrong  direction  as  much  as  in  the  right. 

Hence  such  a  motor  must  run  in  synchronism,  or  not  at  all, 
and  to  operate  properly  it  must  either  be  brought  to  full  syn- 
chronous speed  before  the  alternating  current  is  turned  on,  or 
nursed  into  action  by  running  the  generator  very  slowly,  work- 
ing the  motor  into  synchronous  running  at  very  low  speed, 
and  then  gradually  speeding  up  the  generator,  thus  slowly 
pulling  the  motor  up  to  full  speed.  In  practice  the  former 
method  is  uniformly  employed,  and  the  machine  used  as  a 
synchronous  motor  is  substantially  a  duplicate  of  the  alternat- 
ing generator  as  already  described.    In  fact,  it  is  an  alternating 


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ALTERNATING  CURRENT  MOTORS.  221 

generator  worked  as  a  motor,  just  as  a  continuous  current 
motor  is  the  same  thing  as  the  corresponding  generator. 

Fig.  122  gives  a  clear  idea  of  the  way  in  which  synchnmous 
alternating  motors  may  be  employed  for  power  transmission. 
Here  G  is  the  generator  driven  from  the  pulley  P.  S  is  a 
switch  connecting  the  generator  to  the  line  wires  L  L\  At  the 
motor  end  of  the  line  is  a  second  switch  S\  which  can  connect 
the  line  either  with  the  synchronous  motor  3/,  or  the  starting 
motor  M\  This  latter  is  usually  some  form  of  self-starting 
alternating  motor  to  which  current  is  first  applied,  ii/'  then 
gradually  brings  M  up  to  synchronous  speed ;  when  the  switch 
S'  is  thrown  over,  the  main  current  is  turned  on  M ,  and  then 
the  load  is  thrown  on  the  driving  pulley  P'  by  a  friction  clutch 
or  some  similar  device. 

Such  a  system  has  certain  very  interesting  and  valuable 
properties.  We  can  perhaps  best  comprehend  them  by 
comparing  them  with  the  properties  of  continuous  current 
motor  systems. 

In  the  alternating  system  both  generator  and  motor  are 
usually  separately  excited,  which  means  really  that  the  field 
strengths  are  nearly  constant;  as  constant  in  fact  as  those 
in  a  well  designed  shimt-wound  generator  and  motor  for  con- 
tinuous current. 

Now  we  have  seen  that  this  latter  system  is  beautifully  self- 
regulating.  Whatever  the  load  on  the  motor,  the  speed  is 
nearly  constant,  and  the  current  is  closely  proportional  to  the 
load.  If  the  load  increases,  the  speed  falls  off  just  that  minute 
amoimt  necessary  to  lower  the  count<?r  E.  M.  F.  enough  to 
let  through  sufficient  current  to  handle  the  new  load.  The 
effective  E.  M.  F.  is  the  difference  between  Ej  the  impressed 
E.  M.  F.  and  E'y  the  counter  E.  M.  F.  The  current  produced 
by  this  E.  M.  F.  is  determined  by  Ohm's  law. 

E  -  E' 

C  = (1) 

r 

where  r  is  the  armature  resistance,  and  since  we  have  seen  that 
the  output  of  the  motor  is  measured  by  the  counter  E.  M.  F., 

W^CE'  (2) 


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222 


ELECTRIC  TRANSMISSION  OF  POWER. 


where  W,  in  watts,  includes  frictional  and  other  work.  E', 
neglecting  armature  reaction,  is  proportional  to  the  speed  of 
the  armature,  which  falls  under  load  just  enough  to  satisfy 
equation  (2)  by  letting  through  the  necessary  current. 

Now  we  have  seen  that  when  we  abandon  the  commutator 
the  motor  has  to  run  at  true  synchronous  speed,  or  else  lose 
its  grip  entirely.  How  can  it  adjust  itself  to  changing  condi- 
tions of  load?  If  the  load  increases,  more  current  is  demanded 
to  keep  up  the  output,  but  the  field  strength  remains  constant, 
and  the  counter  E.  M.  F.  of  the  motor  cannot  fall  by  reduction 
of  speed.  We  must  note  that  while  in  a  continuous-current 
motor  the  counter  E.  M.  F.  of  the  armature  is  constant  at 
uniform  speed,  in  an  alternating  motor  the  counter  E.  M.  F. 


Fig.  123. 


varies  like  that  of  the  generator,  following  approximately  a 
sinusoidal  curve,  as  the  position  of  the  armature  with  respect 
to  the  field  poles  varies. 

Hence  at  any  given  instant  the  counter  E.  M.  F.,  the  speed 
and  field  strength  remaining  the  same,  depends  on  the  position 
of  the  motor  armatui-e.  In  Fig.  123  we  have  a  pair  of  alternat- 
ing machines,  generator  A  and  motor  B.  In  normal  running 
at  light  load,  the  two  are  nearly  in  opposite  phase,  since  of 
course  the  impressed  and  counter  E.  M.  F.'s  are  virtually  in 
opposition. 

Now,  if  there  is  an  increase  of  load  the  motor  armature  sags 
backward  a  little  imder  the  strain,  thereby  lessening  the  com- 
ponent of  its  counter  E.  M.  F.  that  is  in  opposition  to  the 


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ALTERNATING  CURRENT  MOTORS.  223 

impressed  E.  M.  F.  The  current  increases,  and  with  it  the 
torque,  and  the  sagging  process  stops  when  the  torque  is  great 
enough  to  carry  the  new  load  as  synchronous  speed.  The 
change  of  phase  in  the  counter  E.  M.  F.  thus  takes  the  place 
of  change  of  absolute  speed  in  the  continuous  current  motor, 
by  the  same  general  process  of  increasing  the  E.  M.  F.  effec- 
tive in  forcing  current  through  the  circuit.  This  efTective 
E.  M.  F.  is  generally  by  no  means  in  phase  with  the  impressed 
E.  M.  F.,  and  in  general  the  current  and  the  impressed  E.  M.  F. 
are  not  in  phase  in  a  synchronous  motor.  Here,  as  elsewhere, 
the  input  of  energy  is 

C  E  cos  4>, 

while  the  output,  which  in  the  continuous  current  motor  is 
simply  the  product  of  the  current  and  the  counter  E.  M.  F., 
in  the  synchronous  motor  depends  evidently  on  such  parts  of 
both  as  are  in  phase  with  each  other,  t.  e,, 

W  =  CE' cos  <l>'  (3), 

in  which  <^  is  the  angle  between  current  and  counter  E.  M.  F. 
Likewise  the  current,  which  in  the  continuous  current  motor 
depends  on  the  effective  E.  M.  F.  and  the  resistance,  now  de- 
pends on  the  counter  E.  M.  F.  and  the  impedance  I.     So  that 

^      E  -  E' 

C  =  — (4). 

In  this  equation  the  values  of  all  the  quantities  depend  on 
their  relative  directions,  and  by  combining  geometrically  the 
factors  of  (4)  we  can  form  a  clear  idea  of  the  singular  relations 
that  may  be  found  in  synchronous  motor  practice. 

The  construction  is  similar  to  that  found  in  Fig.  51,  page  133. 

In  Fig.  124,  we  will  start  with  an  assumed  impressed  E.  M.  F. 
of  1,000  volts,  a  counter  E.  M.  F.  of  800  volts  and  an  impe- 
dance composed  of  5  ohms  resistance  and  10  ohms  equivalent 
inductance. 

To  begin  with,  we  will  lay  off  the  impressed  E.  M.  F.  A  B, 
and  then  the  counter  E.  M.  F.  B  C,  which  as  we  have  seen  is 
in  partial  opposition  to  A  B.  In  this  case  ^1  C  is  the  resultant 
E.  M.  F.,  which,  on  the  scale  taken,  is  300  volts.    This,  then,  is 


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224 


ELECTRIC  TRANSMISSION  OF  POWER, 


the  available  E.  M.  F.  taken  up  by  the  inductive  and  ohmic 

drops  in  the  armature.     The  next  step  is  to  find  C  (eq.  4) 

from  /,  and  the  value  of  E  —  F/,  just  obtained.     To  obtain  /, 

we  must  combine  resistance  and  inductance,  as  shown  in  Fig.  55. 

Performing  this  operation,  it  appeai-s  that  /  =  11.18.     Hence 

300 

in  the  case  in  hand  C  =  -•=  26.8  4-  amperes.     As  to  the 

11.18  ^ 

direction  of  this  current,  we  know  that  it  is  at  right  angles  to  the 

inductive  E.  M.  F.,  i.  c,  is  in  phase  with  the  resistance  in  Fig. 

125.     Solving  that  triangle  to  obtain  the  angle  between  the 

current  and  impedance,  it  turns  out  to  be  a  little  over  63°, 

being  the  angle  whose  tangent  is  —  •    Laying  off  this  angle  a 

5 

from  A  C,  the  impedance  in  Fig.  124,  we  find  the  current  to  be 


FlQ.  124. 

in  the  direction  A  D.    This  current  then  is  out  of  phase  with 

the  impressed  E.  M.  F.  by  the  angle  of  lag  DAB.     It  is  also 

out  of  phase  with  the  counter  E.  M.  F.,  though  by  chance  very 

slightly,  and  lags  behind  the  resultant  E.  M.  F.  A  C,  by  the 

angle  a.     Being  nearly  in  phase  with  the  counter  E.  M.  F.,  the 

gross  output  of  the  motor  is  approximately  26.8  X  800  =  21.4 

KW. 

Now,  what  happens  when  the  load  increases?    The  motor 

armature  sags  back  a  few  degrees  imder  the  added  torque,  and 

the  counter  E.  M.  F.  takes  the  new  position  B  C.    The  new 

resultant  E.  M.  F.  is  A  C,  which  on  the  scale  taken  equals  450 

450 

volts.     The  new  value  of  the  current  is  C  = =  40.25 

11.18 

amperes,  and  its  phase  direction,  63°  from  A  C,  is  A  D',     The 


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ALTERNATING  CURRENT  MOTORS,  226 

new  angle  of  lag  is  then  D'  A  B,  showing  that  under  the  larger 
load  the  power  factor  of  the  motor  has  improved.  If  C  B 
should  lag  still  more,  A  C,  together  with  the  current,  would 
keep  on  increasing.  Evidently,  too,  the  angle  of  lag  D'  A  B 
will  grow  less  and  less  until  A  C  B  becomes  a  right  angle, 
when  in  the  case  shown  it  will  be  very  minute,  and  the  power 
factor  will  be  almost  unity.  Beyond  this  point  the  angle 
CAB  will  obviously  begin  to  decrease,  and  D'  A  B  will  begin 
to  open  out,  again  lowering  the  power  factor  at  very  heavy 
loads. 

Hence  it  appears  that  at  a  given  excitation  there  is  a  par- 
ticular load  for  which  the  power  factor  is  a  maximum,  and  it 
is  evident  from  the  figure  that  in  the  example  taken  this  maxi- 
mum will  be  higher  as  the  inductance  of  the  system  decreases, 


INDUCT  ANCE=10 
Pig.  125. 

and  also  will  pertain  to  a  smaller  output.  Let  us  now  see 
what  happens  when  the  excitation  of  the  motor  is  varied.  In 
Fig.  126  the  conditions  are  the  same  as  before,  except  that  we 
assume  counter  E.  M.  F.'s  of  500  volts  corresponding  to  C  and 
1,100  volts  corresponding  to  C.  Examining  the  former,  the 
resultant  E.  M.  F.  is  A  C  =  528  volts,  the  corresponding  cur- 
rent is  47  +  amperes  and  the  angle  of  lag  D  A  5  is  much 
greater  than  before.  The  power  factor  evidently  would  still 
be  rather  bad  under  increased  loads,  and  worse  yet,  when  at 
lighter  loads  the  angle  ABC  decreases.  Lessened  inductance, 
however,  would  help  the  power  factor  by  decreasing  the  angle 
CAD,  and  hence  BAD,  Now,  consider  the  result  of  in- 
creasing the  motor  excitation  to  5  C  =  1,100  volts.  The 
resultant  E.  M.  F.  now  becomes  A  C,  being  shifted  forward 
nearly  90°,  its  value  is    280  volts  and    the  current  is  25  + 


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226  ELECTRIC  TRAMSMTSSION  OF  POWER, 

amperes.  But  this  current  is  now  in  the  direction  A  /)',  a 
being  the  same  as  before,  and  hence  it  no  longer  lags,  but 
leads  the  impressed  E.  M.  F.  by  nearly  45°.  The  power  fac- 
tor is  therefore  still  bad,  but  gets  better  instead  of  worse 
imder  loads  greater  than  that  showai.  Inductance  in  the 
system  now  improves  the  power  factor,  and  combined  with 
heavy  load  might  bring  the  current  back  into  phase  with  the 
impressed  E.  M.  F. 

The  counter  E.  M.  F.'s  corresponding  to  C  and  C  are 
rather  extreme  cases  for  the  assumed  conditions,  but  it  is  easy 
to  find  a  value  for  the  excitation  which  would  annul  the  lag 
exactly  for  a  particular  value  of  the  load.  Laying  off  in  Fig. 
126,  C  A  B  =^  C  AD'  we  find  the  required  counter  E.  M.  F., 
which  is  very  nearly  910  volts.     At  the  particular  output  cor- 


PlQ.  126. 

responding  to  this  condition,  the  power  factor  is  unity,  the 
current  and  the  impressed  E.  M.  F.  are  in  phase,  and  since 
the  current  is  therefore  a  minimum  for  the  output  in  question, 
the  efficiency  of  the  conducting  system  is  a  maximum.  At 
this  point,  too,  the  energy  is  correctly  measured  by  the  product 
of  volts  and  amperes,  so  that  if  wattmeters  are  not  at  hand 
the  input  at  a  synchronous  motor  can  be  closely  approximated 
at  any  steady  load  by  varying  the  field  until  the  armature  cur- 
rent is  a  minimum,  and  reading  volts  and  amperes. 

Throughout  this  investigation  it  has  been  assumed  that  the 
ratio  of  resistance  and  inductance  has  been  constant.  This  is 
not  accurately  true,  but  is  approximately  so  when  the  induc- 
tance is  fairly  low.  The  phenomenon  of  leading  current  in  a 
synchronous  motor  system  does  not  indicate  that  the  current, 


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ALTERNATING  CURRENT  MOTORS,  227 

in  some  mysterious  way,  has  been  forced  ahead  of  the  E.  M.  F. 
which  produces  it,  for  the  impressed  E.  M.  F.  is  not  responsible 
for  the  current,  which  is  determined  solely  by  the  resultant 
E.  M.  F.  behind  which  the  current  invariably  lags. 

The  net  practical  result  of  all  this  is  that  a  snychronous 
alternating  motor,  under  varying  excitation,  is  capable  of 
increasing,  diminishing,  or  annulling  the  inductance  of  the  sys- 
tem with  which  it  is  connected,  or  can  even  produce  the  same 
result  as  a  condenser  in  causing  the  current  to  lead  the  im- 
pressed E.  M.  F.  The  maximum  torque  of  the  motor,  which 
determines  the  maximum  output,  is  determined  by  the  greatest 
possible  value  of  C  E'  cos  <^'  consistent  with  the  given  im- 
pedance and  electromotive  forces.  The  stronger  the  motor 
field,  and  the  less  the  armature  inductances  and  reactions  of 
both  generator  and  motor,  the  greater  the  ultimate  load  that 
can  be  reached  without  overburdening  the  motor  and  pulling 
it  out  of  step. 

As  regards  the  relation  in  phase  between  current  and  im- 
pressed E.  M.  F.,  the  three  commonest  cases  are  those  for 
which  the  currents  were  computed  for  Figs.  125  and  126.  The 
first,  and  commonly  the  most  desirable,  is  that  in  which  the 
current  lags  slightly  at  small  loads,  gradually  lags  less  and  less, 
comes  into  phase,  or  very  nearly  so,  at  about  average  load,  and 
lags  slightly  again  at  heavy  loads.  The  maximum  efficiency 
of  transmission,  reached  when  the  lag  touches  zero,  is  then  at 
about  average  load.  The  second  and  commoner  case  is  when 
the  motor  is  rather  under-excited,  so  that  the  lag  merely 
reaches  a  rather  large  minimum,  never  touching  zero.  The 
third  case  is  that  in  which  the  current  leads  at  all  moderate 
loads,  passes  through  zero  lag,  and  then  lags  more  and  more. 
The  average  power  factor  may  be  the  same  as  in  the  first  case, 
but  more  energy  is  required  for  excitation,  and  no  advantage 
is  gained  except  in  carrying  extreme  loads,  often  undesirable  on 
account  of  overheating,  or  in  modifying  the  general  lag  factor. 

It  is  highly  desirable  for  economy  in  transmission  that  the 
product  of  current  and  E.  M.  F.  should  be  a  minimum  for  the 
required  load.  This  condition  can  be  fulfilled  for  the  motor 
circuit  at  any  load  by  changing  the  excitation  until  the  current 
for  that  load  becomes  a  minimum.     Further,  the  field  of  a 


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228 


ELECTRIC  TRANSMISSION  OF  POWER, 


uniformly  loaded  motor  may  in  the  same  way  be  made  to  bring 
the  entire  line  current  of  the  system  to  a  minimum  if  the 
motor  be  of  sufficient  capacity.  Thus  a  synchronous  motor 
load  can  be  made  very  useful  in  improving  the  general  condi- 
tions of  transmission.  By  changing  the  motor  excitation  as 
the  load  on  the  motor  of  the  system  varies,  the  power  factor 
can  be  kept  at  or  near  unity  for  all  working  loads. 

Fig.  127  shows  the  power  factor  of  a  synchronous  motor 
somewhat  under-excited,  and  that  of  a  similar  machine  with  a 
field  strong  enough  to  produce  lead  at  moderate  loads.    With 


H  H  H 

PROPORTION  OF  FULL  LOAD 

FlO.  127. 


proper  adjustment  of  its  field,  the  effect  of  a  synchronous 
motor  on  the  general  conditions  of  distribution  is  very  bene- 
ficial. In  curve  A,  Fig.  127,  the  indications  are  that  the  motor 
had  rather  a  high  inductance  and  armature  reaction,  and  the 
excitation  was  decidedly  too  low  for  good  results.  Curve  B 
is  from  a  300  HP  motor,  ^ith  its  field  adjusted  for  zero  lag  at 
about  I  load.  The  inductance  was  low  and  the  armature 
reaction  small.  The  result  is  somewhat  startling.  Even  at  i 
load  the  power  factor  (current  leading)  is  about  .93.  At  half 
load  it  has  passed  .99;  touches  unity,  and  then  slowly  diminishes 


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ALTERNATING  CURRENT  MOTORS, 


229 


to  very  nearly  .98  (lagging)  at  full  load.  In  this  case  the  gen- 
erator was  held  accurately  at  voltage  while  the  excitation  of 
the  motor  was  uniform.  Both  were  polyphase  machines  woimd 
for  2,500  volts. 

When  a  synchronous  motor  is  used  in  this  manner,  it  obvi- 
ously will  show,  at  the  same  load,  values  of  the  current  varying 
if  the  excitation  be  varied.  For  any  load  the  minimum  current 
is  given  by  that  excitation  which  brings  the  current  into  phase 


10  80 

AMPERES  IN  FIELD 
Fio.  128. 


with  the  impressed  E.  M.  F.  This  point  is  fairly  well  defined. 
At  less  excitation  the  current  lags,  with  more  it  leads. 

Fig.  128  shows  for  a  particular  instance  the  relations  between 
the  current  and  the  excitation  of  the  motor  field,  at  full  load 
of  the  motor.  It  is  evidently  easy  to  adjust  the  excitation  to 
the  proper  point. 

In  the  practical  work  of  power  transmission  the  synchronous 
motor  has  several  salient  advantages  to  commend  it.  At  con- 
stant frequency  it  holds  its  speed  absolutely,  entirely  indepen- 
dent of  both  load  and  voltage  until,  from  excessive  load  or 
greatly  diminished  voltage,  it  falls  out  of  phase  and  stops. 


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230  ELECTRIC  TRANSMISSION  OF  POWER. 

It  constitutes  a  load  that  is  substantially  non-inductive,  so 
that  it  causes  no  embarrassing  inductive  complications  in  the 
system,  and  takes  current  almost  exactly  in  proportion  to  its 
work. 

Finally  it  can  be  made  to  serve  the  same  end  as  a  condenser 
of  gigantic  capacity  in  compensating  for  inductances  else- 
where in  the  system,  and  thus  raising  the  general  power  factor 
substantially  to  unity. 

As  compensating  disadvantages,  it  must  run  at  one  fixed 
uniform  speed  imder  all  conditions,  it  is  not  self-starting,  and 
it  requires  the  constant  use  of  a  continuous-current  exciter. 

For  many  purposes  the  fixed  speed  is  no  objection,  and  in 
most  large  work  the  exciter  can  be  used  without  inconvenience. 
Inability  to  start  unaided,  even  when  quite  unloaded,  is  on  the 
other  hand  a  very  serious  matter,  and  has  driven  engineei-s  to 
many  ingenious  subterfuges.  The  simplest  of  these  is  to  pn)-^ 
vide  a  starting  motor,  which  is  supplied  with  power  by  any 
convenient  means,  and  serves  to  bring  the  main  machine  up  to 
synchronous  speed.  Then  the  main  current  is  thrown  on,  the 
motor  falls  into  synchronism,  and  the  load  is  taken  up  by 
means  of  a  clutch.  The  difficulty  is  to  start  the  starting 
motor.  In  transmissions  of  moderate  length,  continuous 
current  may  be  delivered  over  the  main  line  from  the  exciter 
of  the  generator  to  the  exciter  of  the  motor,  which  is  there- 
by driven  as  a  motor,  and  brings  the  alternating  motor  up 
to  speed.  As  the  energy  required  for  this  work  is  not  great, 
say  10  per  cent  of  the  whole  power  transmitted,  it  can  often 
be  delivered  quite  easily.  At  long  distances,  however,  the  drop 
becomes  too  great  for  the  moderate  voltages  available  with 
continuous  current,  and  other  methods  have  to  be  used. 

The  best  known  of  these  is  that  indicated  in  Fig.  122  in 
which  the  synchronous  motor  is  brought  up  to  speed  by  an 
induction  motor  and  thefn  clutched  to  its  load  after  which  the 
induction  motor  is  thrown  out  of  action. 

Another  method  sometimes  used  is  a  special  commutator  to 
rectify  the  current  applied  to  the  main  motor  armature,  thus 
directing  the  impulses  so  as  to  secure  a  small  starting  torque, 
enough  to  bring  the  motor  to  speed.  Then  the  commutator 
is  abandoned  and  the  motor  falls  to  running  synchronously. 


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ALTERNATING  CURRENT  MOTORS. 


231 


An  ingenious  modification  of  this  plan  is  found  in  a  type  of 
self-starting  synchronous  motor  built  by  the  Fort  Wa)nae 
Electric  Corporation,  shown  in  Fig.  129. 

This  machine  has  a  double-wound  armature.  The  main 
winding  is  of  the  kind  usual  in  alternators,  wound  in  slots  in 
the  armature  core,  and  the  leads  belonging  to  it  connect  with 
the  collecting  rings  via  the  bnishes  on  the  pulley  end  of  the 
shaft. 

The  other  winding  is  a  common  continuous  current  drum- 


PlO.  129. 

winding,  laid  uniformly  on  the  exterior  of  the  armature. 
It  is  provided  with  a  regular  commutator  as  shown  in  the 
figure. 

The  field  is  of  laminated  iron,  and  the  field  coils  are  in  dupli- 
cate, there  being  a  coarse  wire  winding  which  in  starting  is  in 
series  with  the  commutated  armature  winding,  and  a  fine  wire 
winding  cut  out  in  starting,  but  used  alone  when  the  motor  is 
at  speed. 

The  motor  in  question  is  started  by  turning  the  alternating 
current,  reduced  to  a  moderate  voltage  by  transformation,  into 


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232  ELECTRIC  TRANSMISSION  OF  POWER. 

the  series  field  and  the  commutated  winding.  The  machine 
then  starts  with  a  good  torque,  and  when  it  has  reached  syn- 
chronous speed,  indicated  by  the  pilot  lamp  on  the  top  of  the 
motor  being  thrown  into  circuit  by  a  small  centrifugal  gov- 
ernor, a  switch  is  thrown  over,  sending  the  main  current 
through  the  alternating  winding  and  closing  the  fine  wire  field 
circuit  upon  the  commutator,  at  the  same  time  cutting  out  the 
series  coils.  The  motor  then  runs  synchronously,  the  excita- 
tion being  furnished  by  the  fine  wire  winding.  This  construc- 
tion is  best  suited  for  rather  small  machines,  as  the  double- 
winding  is  rather  cumbersome  for  large  motors. 

At  present  the  tendency  in  synchronous  motor  practice  is 
wholly  toward  the  use  of  polyphase  machines.  These  will 
start,  when  properly  designed,  as  induction  motors,  or  may  be 
started  by  separate  motors.  When  at  speed  the  field  excita- 
tion is  thrown  on,  and  the  machine  thereafter  runs  in  synchro- 
nism. As  such  motors  in  starting,  as  induction  motors,  take 
a  very  heavy  current  they  are  generally  provided  with  starting 
motors,  although  at  a  pinch  they  may  be  brought  to  speed 
at  reduced  voltage  independently,  especially  if  it  is  practi- 
cable to  dix)p  the  frequency  temporarily  and  thus  to  bring 
generator  and  motor  up  to  speed  together.  Synchronous 
polyphase  motors  possess  the  same  general  properties  as  other 
synchronous  motors,  and  as  most  power  transmission  work 
is  now  done  by  polyphase  currents,  they  are  \\idely  used. 

In  general  transmission  work,  synchronous  motors  find 
their  most  useful  place  in  rather  heavy  work,  which  can  be 
readily  done  at  constant  speed. 

They  have  high  power  factors  even  when  used  for  very 
var}''ing  loads,  and  are  valuable  in  neutralizing  inductance 
in  the  line  and  the  rest  of  the  load.  Even  when  not  deliber- 
ately used  for  this  purpose,  they  raise  the  general  power  factor, 
and  thus  have  a  steadying  effect  that  is  very  useful.  When 
working  imder  steady  load  and  excited  correctly,  they  almost 
eliminate  the  lagging  current  that  sometimes  becomes  so 
great  a  nuisance  in  alternating  current  working. 

The  polyphase  synchronous  motors  will  run  steadily  even 
if  one  of  the  leads  be  broken,  working  then  as  monophase 
machines,  and  by  stiffening  the  excitation  will  generally  carry 


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ALTERNATING  CURRENT  MOTORS.  233 

their  full  normal  loads  without  falling  out  of  synchronism ;  but, 
of  course,  with  increased  heating. 

In  one  case  that  came  to  the  author's  notice,  such  aji  accident 
befell  a  three-phase  synchronous  motor,  which  went  quietly  on 
dri>nng  its  load  of  1,700  looms  for  four  hours,  imtil  the  mill  shut 
down  at  night. 

For  small  motor  work  synchronous  machines  are  somewhat 
at  a  disadvantage,  from  the  complication  of  the  exciter  and 
inability  to  start  under  load.  In  sizes  below  100  HP  they  have 
been  very  generally  superseded  by  the  far  simpler  and  more 
convenient  induction  motor,  the  use  of  which  is  a  most  charac- 
teristic feature  of  modem  power  transmission.  In  the  use  of 
synchronous  motors,  both  monophase  and  polyphase,  there  has 
been  often  encountered  an  annoying  and  sometimes  alarming 
phenomenon  known  as  "hunting,"  or  where  several  machines 
are  involved,  as  "  pumping. "  In  mild  cases  it  appears  merely  as 
a  small  periodic  variation  or  pulsation  of  the  current  taken  by 
the  motor,  often  sufficient  to  cause  embarrassing  periodic  vari- 
ations in  the  voltage  of  the  system.  The  frequency  is  ordi- 
narily one  or  two  periods  per  second,  varying  irregularly  in 
different  cases,  but  being  nearly  constant  for  the  same  machine. 
The  amplitude  may  vary  from  a  few  per  cent  of  the  normal 
current  upwards.  Generally  the  amplitude  remains  nearly 
constant  after  the  phenomenon  is  fairly  established,  but  some- 
times it  sets  in  with  great  violence  and  the  amplitude  rapidly 
increases  until  the  motor  actually  falls  out  of  synchronism. 
This  is  usually  the  result  of  pumping  between  two  or  more 
motors,  and  seems  to  be  especially  serious  in  rotary  convert- 
ers, not  only  throwing  them  out  of  synchronism,  but  throwing 
load  off  and  on  the  generators  ^^dth  dangerous  violence. 

Fig.  130  shows  a  facsimile  of  a  record  from  a  recording  volt- 
meter showing  the  pulsation  of  the  voltage  on  the  system 
produced  by  the  hunting  of  a  300-HP  synchronous  motor.  It 
set  in  as  the  peak  of  the  load  came  upon  the  system  and  per- 
sisted until  the  peak  subsided,  when  it  was  gotten  imder  con- 
trol, only  to  break  out  again  when  the  late  evening  load  fell  off.  » 
During  the  eariy  evening  it  was  so  severe  as  to  produce  pain- 
ful flickering  in  all  the  incandescents  on  the  circuit. 

In  this  case  the  dynamo  tender  was  inexperienced  and  had 


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234 


ELECTRIC  TRANSMISSION  OF  POWER, 


not  acquired  the  knack  of  so  juggling  the  field  current  as  to 
suppress  the  hunting.  A  few  months  later  the  same  system 
was  in  regular  operation  without  the  least  trouble  from  hunt- 
ing, the  operators  by  this  time  having  been  thoroughly  broken 
in.  In  the  majority  of  cases  adroit  variation  of  the  field 
strength  abolishes  hunting,  which  almost  always  starts  with  a 


Fig.  130. 

sharp  change  in  load  or  power  factor.  Just  how  to  handle  the 
excitation  to  obtain  the  best  results  is  a  matter  of  experiment 
in  each  particular  case,  but  except  in  cases  of  unusually  seri- 
ous character  the  knack  is  soon  acquired.  A  rather  strong 
field  often  steadies  things,  although  if  strong  enough  to  pro- 
duce leading  current  the  trouble  is  sometimes  aggravated. 


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ALTERNATING  CURRENT  MOTORS.  236 

But  iji  this  case,  as  in  operating  alternators  in  parallel, 
the  best  running  conditions  have  to  be  learned  by  expe- 
rience. 

Much  yet  remains  to  be  learned  about  the  exact  nature  of 
hunting,  but  its  general  character  is  about  as  follows:  A 
sudden  change  in  current  or  phase  causes  the  armature  to  seek 
a  new  position  of  equilibrium.  In  so  doing  the  sudden  change 
in  the  armature  reaction  momentarily  changes  the  field 
strength,  which  aggravates  the  instabilty  already  existing  and 
causes  the  armature  imder  the  influence  of  its  own  inertia  to 
overreach  and  run  beyond  its  normal  position  of  equilibrium. 
Then  the  field  recovers  and  the  armature  swings  back,  once 
more  shifting  the  field  and  again  overrimning,  and  so  on  ad 
nauseam.  The  pulsation  of  the  exciting  current  in  cases  of 
hunting  is  generally  very  conspicuous,  and  the  periodicity  of 
the  himting  seems  to  correspond  in  general  with  the  time  con- 
stant of  the  field  magnetization. 

A  fly-wheel  on  the  motor  or  direct  connection  to  a  heavy 
machine  generally  increases  the  trouble  by  increasing  the 
mechanical  momentum  of  the  armature,  while  belted  and  flex- 
ibly cozmected  motors  suffer  less.  Heavy  drop  in  the  supply 
lines,  which  makes  the  voltage  at  the  motor  sensitive  to  varia- 
tions of  current,  and  low  reactance  in  the  armature,  which 
favors  large  fluctuations  of  current,  are  conditions  specially 
favorable  to  violent  hunting.  Rotaiy  converters  in  which  the 
armature  current  and  its  reactions  are  very  heavy,  compared 
with  that  component  of  the  current  which  is  directly  concerned 
with  the  rotation  of  the  machine  as  a  synchronous  motor,  are 
subject  to  peculiarly  vicious  hunting,  which  has  often  risen  to 
the  point  where  it  threw  the  rotary  out  of  synchronism. 
They  are  far  less  stable  in  this  particular  than  ordinary  syn- 
chronous motoi-s,  and  cannot  readily  be  controlled  by  varying 
the  excitation  on  account  of  the  consequent  variation  of  vol- 
tage on  the  continuous  current  side. 

Motors  and  rotaries  having  their  pole  pieces  not  laminated, 
but  solid,  often  show  less  tendency  to  hunt  than  machines  with 
laminated  poles.  If  the  poles  are  solid  any  violent  swaying  of 
the  armature  current  with  reference  to  them  is  checked  and 
damped  by  the  resulting  eddy  currents,  so  that  the  himting  is 


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236 


ELECTRIC  TRANSMISSION  OF  POWER. 


pretty  effectively  choked.  For  the  same  reason  alternators  in 
parallel  are  less  likely  to  pump  if  they  have  solid  poles,  and  most 
foreign  machines  are  built  in  such  wise.  Here,  where  laminated 
poles  are  just  now  the  rule,  recourse  is  had  to  ** bridges"  or 
"shields."  These  are  essentially  heavy  flanges  of  copper  or 
bronze  attached  to  the  edges  of  the  poles,  so  that  fluctuations  of 
armature  reaction  and  of  field  are  damped  by  heavy  eddy  cur- 
rents whenever  they  arise,  the  bridges  acting  indeed  like  a 
rudimentary  induction  motor  winding.  An  example  of  such 
practice  is  shown  in  Fig.  131,  which  shows  a  portion  of  the 


FlO.  131. 

revolving  field  of  a  large  polypnase  machine  fitted  with  mas- 
sive castings,  bridging  the  spaces  from  pole  piece  to  pole  piece 
and  serving  at  once  to  hold  the  field  coils  rigidly  wedged  into 
place  and  to  check  pumping.  A  similar  device  is  used  in  con- 
nection with  many  rotary  converters  with  a  very  fair  degree 
of  success.  Occasionally  pumping  may  be  traced  to  some 
definite  cause  like  a  defective  engine  governor  having  a  periodic 
vibration,  but  more  often  the  phenomenon  is  purely  electro- 
magnetic. The  use  of  shields  or  solid  pole  pieces  constitutes 
the  best  general  remedy,  for,  while  adjustment  of  the  field 
is  often   effective,  it   is   often   desirable  to  adjust   the  field 


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ALTERNATING  CURRENT  MOTORS,  237 

for  other  purposes,  and  the  necessity  of  varying  it  to 
suppress  hunting  is  sometimes  very  embarrassing,  if  not 
impossible.* 

INDUCTION   MOTORS. 

An  induction  motor  is  a  motor  into  which  working  current 
is  introduced  by  electromagnetic  induction  instead  of  by 
brushes.  It  has  therefore  two  distinct,  although  coordinated, 
fimctions  —  transformer  and  motor.  To  understand  its  action 
we  must  take  care  not  to  confuse  these  fimctions,  and  this  is 
best  done  by  recurring  to  the  fundamental  principles  that  are 
at  the  root  of  all  motors  of  whatever  kind. 

An  electric  motor  consists  of  these  essential  parts,  viz.:  A 
magnetic  field,  a  movable  system  of  wires  carrying  electric 


FlO.  132. 

currents,  and  means  for  organizing  these  two  elements  so  as 
to  produce  continuous  torque. 

These  parts  are  beautifully  shown  in  their  elementary 
simplicity  in  Barlow's  wheel,  Fig.  132,  invented  some  three- 
quarters  of  a  century  ago. 

In  this  machine  A^  S  is  the  permanent  field-magnet,  the 
arms  of  the  star-shaped  wheel  are  the  current-carrying  con- 
ductors, and  a  little  trough  placed  between  the  magnet  poles, 
and  partly  filled  with  mercury,  serves  with  the  wheel  as  a  com- 
mutator. Its  function  is  to  shift  the  current  from  one  con- 
ductor to  the  next  following  one,  when  the  first  passes  out  of 
an  advantageous  position.  In  other  words  it  keeps  the  cur- 
rent flowing  so  as  to  produce  a  continued  torque,  irrespective 
of  the  movement  of  the  conductors.     Such  is  precisely  the  Timc- 

*  For  the  mathematical  theory  of  the  sabject  see  Steinmetz,  Trans. 
A.  I.  E.  E.  Hay,  1002. 


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238 


ELECTRIC  TRANSMISSION  OF  POWER, 


tion  of  the  modem  commutator,  and  it  is  interesting  to  note 
that  the  device  of  making  the  armature  conductors  themselves 
serve  as  the  commutator  is  successfully  used  in  some  of  the 
best  modem  machines. 

These  same  fundamental  parts  are  found  alike  in  motors 
designed  for  continuous  or  for  alternating  currents.  We  have 
already  seen  that  a  series-wound  motor  can  serve  for  use  with 
both  kinds  of  current,  since  the  commutator  distributes  the 
current  alike  for  both,  and  since  the  direction  of  the  torque  is 
determined  by  the  relative  direction  of  the  main  field  and  that 
due  to  the  moving  conductors,  alternations  which  affect  both 
symmetrically  leave  the  torque  unchanged. 


Fio.  138. 

We  have  seen  also  that  if  the  distribution  of  currents  given 
by  the  commutator  can  be  simulated  by  supplying  the  arma- 
ture with  alternating  impulses  timed  as  the  commutator  would 
time  them,  we  can  dispense  with  the  commutator,  and  sub- 
stitute two  slip  rings.  In  this  case,  however,  the  motor  will 
run  only  when  in  synchronism,  since  then  only  will  the  alternat- 
ing impulses  from  the  generator  be  properly  distributed  in  the 
armature,  as  has  already  been  explained.  Besides,  the  current 
has  to  be  introduced  into  the  armature  through  brushes  bear- 
ing on  a  pair  of  slip  rings,  and  an  exciter  is  required  to  supply 
the  field.  If  one  could  use  an  alternating  field,  and  induce  the 
currents  in  the  armature  as  one  would  in  the  secondary  of  a 
common  transformer,  the  machine  would  be  of  almost  ideal 
simplicity. 


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ALTERNATING  CURRENT  MOTORS. 


289 


This  is  what  is  accomplished  in  the  induction  motor.  The 
field  is  supplied  with  alternating  current,  and  the  working  cur- 
rent is  induced  directly  in  the  armature  conductors. 

To  this  end  the  brushes  used  in  the  previous  examples  may 
be  replaced  by  a  pair  of  inducing  poles,  carrying  the  primary 
windings,  to  which  the  armature  windings  play  the  r61e  of 
secondary.  These  armature  windings  are  therefore  closed 
on  themselves,  instead  of  being  brought  out  to  slip  rings. 

For  this  short-circuited  winding  various  forms  are  employed, 
the  simplest  being  shown  in  Fig.  133.  It  consists  of  a  set  of 
copper  bars  thrust  through  holes  near  the  periphery  of  the 


Fio.  134. 

laminated  armature  core,  and  all  connected  together  at  each 
end  by  heavy  copper  rings. 

The  simplest  arrangement  of  field  and  inducing  poles  is 
shown  in  Fig.  134.  Here  each  pair  of  opposite  poles  is  provided 
with  a  separate  winding,  so  that  the  circuit  A  A  supplies  alter- 
nating current  to  one  pair  and  B  B  to  the  other  pair.  The 
armature  we  will  assume  to  be  like  Fig.  133.  Now  apply  an 
alternating  current  to  A  A,  The  windings  of  the  armature 
which  enclose  the  varying  electromagnetic  stress  will  have  set 
up  in  them  a  powerful  alternating  current  almost  180°  behind 
the  primary  current,  i.e.,  in  general  opposed  to  it  in  direction,  as 
considerations  of  energy  require.    The  armature  will  not  turn, 


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240  ELECTRIC  TRANSMISSION  OF  POWER. 

however,  for  two  very  good  reasons:  first,  the  current  in  it  is 
far  out  of  phase  with  the  magnetization  of  the  poles;  and 
second,  this  current  is  quite  symmetrical  w^ith  respect  to 
the  poles,  so  that  the  only  effect  could  be  a  straight  push  or 
pull  without  the  slighest  tendency  to  attract  or  repel  one 
side  of  the  armature  more  than  the  other. 

To  produce  rotation  as  a  motor,  there  must  be  not  only  a 
current  in  the  armature  conductors,  but  there  must  be  field 
poles  magnetized  and  disposed  so  as  to  produce  a  torque  upon 
these  conductors. 

Suppose,  now,  an  alternating  current  to  be  sent  around  the 
circuit  B  B.  If  it  is  applied  simultaneously  with  the  current 
in  A  il,  we  shall  be  no  better  off  than  before,  for  since  the  two 
pairs  of  poles  act  together  and  just  alike,  there  is  no  magnetiza- 
tion in  phase  with  the  armature  current,  and  nothing  to  cause 
the  armature  to  turn  either  way. 

To  obtain  rotation  we  must  arrange  the  two  sets  of  poles  so 
that  one  pair  may  furnish  a  magnetic  field  with  which  the  cur- 
rent induced  by  the  other  pair  is  able  to  react.  The  simplest 
way  of  doing  this  is  to  supply  B  B  with  current  90^  in  phase 
behind  the  current  in  A  A.  Then  when  the  current  induced 
hy  A  A  rises,  it  finds  the  poles  B  B  energized  and  ready  to 
attract  it,  for  the  magnetization  in  B  B  and  the  current  are 
less  than  90^  apart  in  phase.  The  less  the  lag  of  the  arma- 
ture current  behind  its  E.  M.  F.,  the  more  nearly  will  the 
magnetization  of  these  field  poles  be  in  phase  with  the  armature 
current,  and  the  more  powerful  will  be  the  torque  produced. 

The  B  B  set  of  poles  necessarily  induce  secondary  currents  in 
the  armature  in  their  turn,  toward  which  the  A  A  poles  serve 
as  field  during  the  next  alternation.  The  directions  of  both 
armature  current  and  field  magnetization  are  now  reversed, 
so  that,  as  in  the  commutating  motor,  the  torque  is  im changed. 
The  next  alternation  begins  the  cycle  over  again,  and  so  the 
motor  runs  up  to  speed.  Its  direction  of  rotation  depends 
evidently  upon  the  relative  directions  of  magnetization  in  the 
two  sets  of  poles,  for  these  determine  the  direction  of  the 
armature  current  and  the  nature  of  the  field  poles  that  act 
upon  it.  Reversing  the  current  in  A  -4  or  B  B  will  therefore 
reverse  the  motor,  while  reversing  both  will  not. 


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ALTERNATING  CURRENT  MOTORS.  241 

The  speed  of  the  armature  is  determined  in  a  rather  inter- 
esting manner.  When  the  armature  is  in  rotation  the  electro- 
magnetic stresses  which  act  upon  a  given  set  of  armature 
conductors  are  subject  to  variation  from  two  causes.  First  is 
the  variation  in  magnetization,  due  to  changes  in  the  primary 
current;  second,  the  variation  due  to  the  armature  coils  mov- 
ing as  the  armature  turns,  so  as  to  include  more  or  less  of  the 
magnetic  stress.  The  E.  M.  F.  in  the  armature  conductors  is 
due  to  the  summed  effect  of  these  two  variations.  And  since 
the  two  are  in  opposition,  if  the  armature  were  moving  fast 
enough  to  make  a  half  revolution  for  each  alternation  of  the 
field,  the  E.  M.  F.  produced  would  be  zero,  since  the  rates  of 
change  in  the  field  and  in  the  area  of  stress  included  by  the 
armature  coils  would  be  equal. 

This  means  that  the  armature  must  always  run  at  less  than 
synchronous  speed  —  enough  less  to  produce  a  net  armature 
E.  M.  F.  high  enough  to  give  sufficient  armature  current  for 
the  torque  needed. 

Under  varying  loads,  therefore,  an  induction  motor  behaves 
much  Uke  a  shimt-wound  continuous  current  motor.  In  both, 
the  armature  current  is  due  to  the  net  effect  of  an  applied  and 
a  coimter  E.  M.  F.,  the  former  being  delivered  from  the  line 
through  brushes  in  the  one  case  and  by  induction  in  the 
other.  In  neither  case  can  the  speed  rise  high  enough  to 
equalize  these  two  E.  M.  F.'s.  There  is,  however,  a  very  curi- 
ous and  interesting  form  of  induction  motor  which  runs  at 
tnie  synchronous  speed  until  the  load  upon  it  reaches  a  certain 
point,  when  it  falls  out  of  step  like  any  other  synchronous 
motor,  or  under  certain  circumstances  falls  out  of  synchronism 
and  then  operates  like  an  ordinary  asynchronous  motor. 

Its  operation  in  synchronism  seems  a  paradox  at  the  first 
glance;  but  the  principle  involved  is  really  simple,  although 
the  exact  theory  of  the  motor  is  a  bit  complicated.  As  has 
already  been  noted,  if  the  rates  of  change  of  magnetic  induc- 
tion due  to  the  pulsation  of  the  field  and  to  the  cutting  of 
the  field  by  the  armature  coils  are  equal  and  opposite,  there 
will  be  no  E.  M.  F.  in  these  coils,  and  obviously  no  energy 
can  be  transferred  from  field  to  armature.  If,  however,  the 
E.  M.  F.  wave  due  to  the  change  of  magnetization  in  the  field 


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242  ELECTRIC  TRANSMISSION  OF  POWER. 

and  that  due  to  the  motion  of  the  armature  coils  through  the 
field  are  very  diflferent  in  shape,  there  can  still  be  a  periodical 
resultant  E.  M.  F.,  generally  of  a  very  complicated  description, 
accompanied  by  a  transfer  of  energy  even  at  full  synchronous 
speed.  A  very  irregular  wave  shape  in  the  E.  M.  F.  of  supply, 
or  a  distortion  of  it  due  to  extraordinary  armature  reactions, 
may  produce  this  condition.  Fig.  135  shows  the  primary 
E.  M.  F.  wave  form  as  taken  by  the  oscillograph  across  the 
terminals  of  such  a  synchronous  induction  motor,  and  the  cor- 
responding current  wave,  which  emphasizes  the  significance  of 
the  facts  just  given.  The  condition  is  best  reached  in  small 
motors  having  sharply  salient  field  poles.  The  writer  has 
never  seen  one  which  would  start  from  rest  unaided,  the  great 


PlO.  135. 


field  distortion  necessary  being  in  the  way,  but  once  spun  up 
to  or  near  synchronism  they  work  admirably  on  a  small  scale. 
The  conditions  of  energy  supply  are  obviously  such  as  to  be 
highly  unfavorable  in  motors  of  any  size,  but  for  laboratory  or 
other  purposes  where  synchronous  speed  is  wanted  they  are 
very  convenient  for  an  output  of  i  HP  or  so,  and  form  a 
very  striking  modification  of  the  ordinary  induction  motor. 
They  have,  up  to  the  present,  been  made  mostly  by  the  Holt- 
zer-Cabot  Electric  Co. 

If  the  load  on  an  induction  motor  increases,  demanding  an 
increased  torque,  the  armature  slows  down  a  trifle,  until  the 
new  armature  E.  M.  F.  and  resulting  current  are  just  sufficient 
to  meet  the  new  conditions.  In  the  continuous  current  motor 
this  speed  is  determined  by  the  resistance  of  the  armature,  to 
which  the  current  corresponding  to  a  given  decrease  of  speed 


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ALTERNATING  CURRENT  MOTORS. 


243 


is  necessarily  proportional.  In  the  induction  motor  the  arma- 
ture resistance  plays  a  precisely  similar  r61e.  Fig.  136  shows 
the  actual  speed  variation  of  a  100  HP  induction  mbtor  in  terms 
of  its  output.  The  maximum  fall  in  speed  under  full  load  is  a 
trifle  less  than  3  per  cent,  and  even  this  result  is  sometimes 
surpassed  in  induction  motors  for  especial  purposes,  even  a  1 
per  cent  variation  having  been  reached.  A  motor  with  higher 
armature  resistance  w^ould  fall  more  in  speed,  like  a  shunt 
motor  with  a  rather  high  armature  resistance.  We  thus  see 
that  the  induction  motor,  as  it  should,  behaves  much  like  any 
other  motor;  the  torque  is  produced  in  the  same  way,  and 
obeys  similar  laws;  the  motor  is  similarly  self-starting,  and 
works  on  the  same  general  principles  throughout.  Obviously 
the  magnitude  of  the  annature  current  in  an  induction  motor  is 


s 

'^A  ■^'  ^»^a 

6ft       ^ 

il 

y 

•  HP.  OUTPUT 
Flo.  136. 


determined,  not  by  the  armature  resistance  alone,  but  by  its 
impedance.  As,  however,  the  presence  of  reactance  shifts  the 
phase  of  the  current,  and  that  component  of  current  which  is 
effective  in  producing  torque  depends  upon  the  resistance, 
the  relation  just  explained  holds  good.  That  current  is  deliv- 
ered to  the  armature  by  induction  is  a  striking  feature,  but 
not  one  that  implies  any  radical  difference  in  principle. 

It  is  not  even  necessary  to  use  a  polyphase  circuit  for  work- 
ing induction  motors,  for,  imder  certain  conditions,  the  same 
set  of  poles  can  perform  the  double  duty  of  delivering  current 
and  interacting  with  it  to  produce  torque. 

The  principles  of  the  induction  motor,  as  here  given,  thus 
become  part  of  the  general  theory  of  the  electric  motor  which 
applies  aHke  to  machines  for  continuous  and  alternating  cur- 
rent, quite  independent  of  particular  methods  of  construction 
or  operation. 


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244 


ELECTRIC  TRANSMISSION  OP  POWER, 


The  great  pioneers  in  induction  motor  work,  Tesla,  Ferraris, 
and  some  others,  preferred  to  view  the  matter  from  the  special 
rather  than  the  general  standpoint,  and  hold  to  the  theory  of 
the  rotary  pole  action  of  induction  motors  —  very  beautiful, 
mathematically,  but  unfortunately  hiding  the  Icinship  of  induc- 
tion to  other  motors,  and  distracting  attention  from  the  trans- 
former action,  which  is  so  prominent. 

From  this  point  of  view  the  two  pairs  of  poles  in  Fig.  134 


FIG.  187. 


Fia.  138. 


Pig.  139. 


co-act  to  produce  an  oblique  resultant  magnetization,  which 
shifts  around  the  field^  producing  a  iijoving  system  of  poles, 
following  the  sequence  of  the  current  phases,  and  dragging 
around  the  armature  after  them,  by  virtue  of  the  currents  in- 
duced in  it.  Figs.  137,  138,  139  show  the  rudimentary  prin- 
ciples of  the  rotary  pole.  In  Fig.  137  an  annular  field  magnet 
is  wound  with  two  circuits  A  A  and  B  B,  supplied  with  alter- 
nating currents  90®  apart  in  phase.  The  polarity  of  the 
armature  is  represented  diagrammatically  by  the  rotating 
magnet  A^  S. 


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ALTERNATING  CURRENT  MOTORS.  245 

Now,  when  the  current  in  il  il  is  maximum  (and  that  in  J5  B 
is  consequently  zero),  the  field  has  poles  at  P  and  P',  which 
exert  a  torque  on  the  armature  poles.  As  the  current  falls  in 
A  A  and  rises  in  B  B,  the  resultant  poles  move  forward  to  P^ 
and  P/  (Fig.  138),  followed  by  the  armature.  When  the  cur- 
rent BBisA  maximum,  and  A  A  has  become  zero,  the  poles  are 
at  P2  and  P,'  and  so  on.  In  order  that  the  revolving  poles 
may  induce,  current  in  the  armature,  the  latter  must  slip 
behind  so  as  to  produce  relative  motion  and  change  in  electro- 
magnetic stress. 

This  point  of  view  is  very  interesting  and  instructive.  It 
deals,  however,  not  directly  with  the  two  field  magnetizations 
—  the  functions  of  which  have  just  been  discussed  —  but  with 
a  resultant  rotary  magnetic  field,  which  may  or  may  not  have 
a  concrete  existence,  according  to  circumstances.  It  by  no 
means  follows  that  because  two  equal  energizing  currents  are 
90®  apart  in  phase,  they  must  or  do  form  a  resultant  rotary 
magnetic  field,  or  that,  if  they  are  so  organized  as  to  give  a 
ph3rsical  resultant,  their  individual  functions  are  superseded 
and  must  be  neglected. 

The  two  views  of  the  induction  motor  here  set  forth  are  not 
in  any  way  conflicting;  they  merely  represent  two  methods  of 
treatment  of  the  same  phenomena.  As  it  happens,  the  rotary 
field  point  of  view  is  from  a  mathematical  standpoint  the 
easier,  for  it  treats  the  resultant  instead  of  its  components, 
and  hence  has  been  the  oftener  used,  but  in  discussing  certain 
classes  of  induction  motors,  it  is  by  no  means  convenient,  and 
is  less  general  than  the  analytical  method,  which  deals  with  the 
separate  components.  In  most  commercial  induction  motors 
there  is  imdoubtedly  a  resultant  rotary  field,  but  however  con- 
venient it  may  be  to  consider  the  motors  in  that  light,  it  is  not 
well  to  lose  sight  of  the  general  actions  of  which  the  rotary 
field  is  a  special  case. 

As  a  matter  of  fact,  the  several  currents  in  a  polyphase  in- 
duction motor  may  be  so  distributed  that  they  cannot  produce 
a  resultant  rotary  magnetization,  and  in  certain  heterophase 
and  monophase  motors  the  "rotary  field,"  in  so  far  as  one  is 
formed  by  the  field,  may  revolve  in  one  direction  while  the 
armature  starts  and  runs  strongly  in  the  other  direction. 


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246 


ELECTRIC  TRANSMISSION  OF  POWER, 


Hence,  the  view  here  taken  of  the  induction  motor  has  been 
generalized  for  the  purpose  of  bringing  out  its  relation  to  the 
general  theory  of  motors,  and  to  take  account  of  induction 
motors,  in  explaining  which  the  rotary  pole  theory  would  have 
to  be,  as  it  were,  dragged  in  by  the  ears. 

Salient  poles,  like  those  of  Fig.  134,  are  seldom  used,  and 
the  induction  motor  as  generally  constructed,  consists  of  two 


FlO.  140. 


short  concentric  cylinders  of  laminated  iron,  slotted  on  their 
opposed  faces  to  receive  the  windings.  Sometimes  these  slots 
are  open,  and  again  they  are  simply  holes  close  to  the  surface 
of  the  iron. 

The  relation  of  the  parts  is  well  shown  in  Fig.  140,  a  6  HP 
two-phase  motor  by  C.  E.  L.  Brown. 

In  this  case  the  exterior  ring  is  the  primary,  and  the  revol- 
ving ring  the  secondary  element  of  the  motor.  The  primary 
^^dnding  is  of  coils  of  Gne  wire  threaded  through  the  core 
holes,  while  the  secondary  member  is  wound,  if  one  may  use 


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ALTERNATING  CURRENT  MOTORS. 


247 


the  term,  with  solid  copper  rods  imited  at  the  ends  by  a  broad 
copper  ring.  The  clearance  between  primary  and  secondary 
is  very  small  in  all  induction  motors,  almost  always  less  than 
I  inch,  sometimes  less  than  ^  inch.  The  smaller  the  clear- 
ance the  better  the  machine  as  a  transformer. 

The  primary  of  an  induction  motor  is  wound  much  as  the 
armature  of  a  polyphase  generator  is  wound,  as  described 
already.  Fig.  141  shows  in  diagram  a  two-phase  winding  for  a 
24  slot  primary,  and  Fig.  142  a  three-phase  winding  for  the 
same  primary.    In  the  former  there  are  two  sets  of  coils,  A 


Fig.  141. 

and  B,  each  forming  a  separate  phase  winding;  in  the  latter  the 
three  sets,  A,  B,  d  may  be  united  to  form  either  a  "star"  or 
^'mesh"  three-phase  winding.  In  practice  the  primary  winding 
is  nearly  always  polydontal,  for  the  same  general  reasons  that 
hold  for  generator  armatiu*es,  but  especially  to  keep  down 
inductance.  For  the  same  reason  the  secondary  winding  is 
polydontal.  As  an  example  of  the  best  usage  in  this  respect, 
Rg.  145  shows  the  number  and  relation  of  primary  and 
secondary  slots  in  the  motor  shown  in  Fig.  140.  There  are  no 
less  than  40  primary  slots  for  a  four-pole  winding,  t.6.,  5  slots 
per  phase  per  pole,  while  the  secondary  has  37  slots,  this  od<J 


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ELECTRIC  TRANSMISSION  OF  POWER. 


number  being  chosen  to  reduce  the  variation  in  the  magnetic 
relations  of  primary  and  secondary  due  to  diflferent  positions 
of  the  armature. 

Induction  motors  with  fixed  primary  have  the  great  advan- 
tage of  having  no  moving  contacts,  and  no  liigh  voltage  wind- 
ings exposed  to  the  strains  due  to  revolution.  On  the  other 
hand  a  revolving  primary  makes  it  very  easy  to  vary  the  resist- 
ance in  the  secondary  circuit,  which  is  often  desirable.  Both 
forms  are  used,  the  latter  only  rarely.  Inasmuch  as  a  large 
proportion  of  the  hysteretic  loss  occurs  in  the  primary,  since 


FlO.  142. 

in  the  secondary  the  variation  of  the  magnetization  is  small, 
a  revolving  primary,  being  of  less  dimensions  than  its  secon- 
dary, gives  a  slight  advantage  in  efficiency.  There  is,  however, 
small  reason  to  suppose  that  on  the  whole  it  is  easier  to  build 
one  form  than  the  other  for  a  given  efficiency  with  the  same 
care  in  designing..  In  recent  practice  it  is  not  imcommon  to 
wind  the  primaries  of  large  induction  motors  for  voltages  up 
to  10,000. 

Motors  with  revolving  primary  are  no  longer  regularly  man- 
ufactiu^d,  the  vastly  superior  simplicity  of  the  other  con- 
struction being  generally  recognized.    Plate  ^"11  shows  the 


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Fig.  1. 


FlO.  X 


PLATE  Vn. 


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ALTERNATING  CURRENT  MOTORS. 


249 


motors  now  in  common  use  in  this  country.  Fig.  1  is  a  stan- 
dard Westinghouse  "Type  C  C  "  motor  of  10  HP.  It  is  exceed- 
ingly simple  in  construction,  and  efficient  in  operation.  It  has 
a  "squirrel-cage"  armature  similar  to  that  of  Fig.  133,  but 
the  bars  are  in  open  slots  and  are  of  rectangular  section,  a 
construction  which  gives  a  lower  armature  reactance  than  if 
the  iron  were  closed  over  the  armature  bars.  These  motors 
start  with  a  powerful  torque,  approximately  two  or  three 
times  the  torque  at  rated  full  load,  when  the  full  line  voltage 
is  thro^sTi  upon  the  primary,  but  of  course  take,  under  these 
conditions,  a  very  heavy  current,  so  that  in  practice  it  is  usual 
to  start  them  at  reduced  voltage,  which  gives  all  the  torque 


necessary  without  calling  for  excessive  current.  This  is  ac- 
complished by  means  of  a  so-called  auto-converter,  of  which 
the  essential  connections  are  shown  in  Fig.  143.  With  the 
switch  in  the  starting  position  the  applied  voltage  is  only  a 
quarter  or  a  half  the  normal  voltage,  the  actual  amount  being 
adjusted  by  means  of  the  variable  connections  sho'WTi,  and 
when  the  motor  has  come  up  to  its  full  speed  under  the  starting 
conditions  the  switch  is  suddenly  thrown  over,  putting  the  full 
working  voltage  in  circuit.  The  actual  appearance  of  the 
latest  form  of  auto-starter  is  shown  in  Fig.  144.  The  starting 
voltage  is  applied  in  several  steps  and  the  whole  device  is 
immersed  in  oil.    It  is  necessary  to  let  the  motor  reach  its  full 


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ELECTRIC  TRANSMISSION  OF  POWER. 


speed  with  the  lower  voltage  before  making  this  change,  else 
there  will  be  a  needlessly  severe  current  due  to  the  sudden 
acceleration  under  full  voltage,  and  the  change  should  be 
made  quickly,  lest  the  armature  speed  should  fall  oflF  during 
the  change  and  produce  the  same  unpleasant  result.  When 
intelligently  handled  the  starting  current  can  be  kept  within 
very  reasonable  limits,  but  the  auto-converter  should  be  ad- 
justed when  set  up  to  give  at  starting  merely  the  voltage 
needed  to  start  under  the  required  torque,  an  excess  of  voltage 
meaning  excess  of  cun-ent.     Fig.  2  of  Plate  VII  is  a  General 


FlO.  U4. 

Electric  **Type  L"  motor  of  35  HP.  The  mechanical  design 
is  very  simple,  giving  a  light  and  well  ventilated  structure. 
The  bearings  can  be  shifted  to  compensate  for  wear.  The 
winding  of  the  armature  is  a  regular  three-phase  bar  winding 
furnished  with  starting  resistances  within  the  spider,  which  are 
cut  out  gradually  by  means  of  a  ring  moved  by  the  lever  seen 
just  within  the  bearing  spider.  The  starting  resistances  are 
in  many  sections  and  can  be  short-circuited  very  gradually, 
holding  the  primary  current  practically  constant  from  start  to 
full  speed,  even  when  starting  \mder  a  heavy  torque.     The 


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ALTERNATING  CURRENT  MOTORS. 


251 


start  is  made  with  the  lever  pulled  out  to  its  fullest  extent, 
and  it  is  gradually  pushed  home  luitil  full  speed  is  reached. 
Such  motors  are  peculiarly  well  adapted  for  use  on  lighting 
circuits,  and  in  large  sizes  requiring  heavy  starting  torque. 
The  start  can  be  made  with  very  moderate  currents,  and  the 
torque  per  ampere  is  considerably  greater  than  in  any  motor 
starting  on  reduced  primary  voltage,  which  is  the  compensa- 
tion for  the  rather  elaborate  starting  device.     Neither  of  the 


FIO.  146. 

companies  mentioned  holds  rigidly  to  the  constructions  here 
shown,  but  the  cuts  show  their  best  standard  practice.  There 
is  very  little  difference  in  the  essential  properties  of  the  two 
fonns,  and  both  are  very  widely  used. 

These  recent  motors  are  nearly  all  made  with  extremely 
small  clearance  between  armature  and  field,  from  -Ar  to  A  inch 
or  less,  even  in  large  motors.  This  practice  renders  it  easy  to 
design  for  a  good  power  factor,  but  may,  and  sometimes  does, 
cause  trouble  mechanically,  as  might  be  anticipated.  It  is  not 
difl&cult  to  make  thoroughly  good  motors  without  resorting  to 


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252 


ELECTRIC  TRANSMISSION  OF  POWER, 


such  extreme  measures,  unless  the  designer  is  hampered  by 
troublesome  specifications  in  other  particulars.  Demand  for 
slow  speed  motors  at  a  periodicity  of  60^,  and  insistence  on  a 
uniformity  of  speed  at  various  loads  that  would  not  for  a  mo- 
ment be  demanded  in  direct  current  motors,  are  responsible 
for  serious  and  needless  impediments  in  induction  motor . 
design. 

The  "Type  L'*  motors  just  described  have  on  the  armature 


Fia.  146. 

a  regTilar  three-phase  winding  of  rectangular  Jjjars  united  by 
end  connectors.  A  simple  fotu'-pole  form  of  such  a  winding  is 
shown  in  Fig.  146.  It  obviously  is  more  troublesome  to  con- 
struct than  a  "squirrel  cage"  "s^inding,  but  it  possesses  certain 
advantages.  Conspicuously,  it  renders  it  possible  to  insert  the 
resistance  in  the  secondary  circuit  at  starting,  which  in  the 
"squirrel  cage"  would  be  a  very  difficult  matter,  although  it 
has  been  tried. 


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ALTERNATING  CURRENT  MOTORS. 


258 


If  the  field  is  very  uniform,  with  a  thoroughly  distributed 
winding,  there  is  very  Httle  difference  in  the  actual  perform- 
ance of  the  two  kinds  of  armatures  (drum-wound  and  "squir- 
rel cage")  when  at  speed.  In  case  of  a  motor  with  salient 
poles  or  with  few  winding  slots  in  the  field,  the  drum  armature 
has  a  very  considerable  advantage,  owing  to  the  fact  that  the 
currents  in  it  are  directed  into  definite  paths  which  they  must 
follow  at  ail  times,  while  in  the  ''squirrel  cage"  form  the  cur- 
rents are  only  uniformly  organized  when  there  is  a  imiform 
field.     In  the  early  motors,  therefore,  the  drum- wound  arma- 


FlQ.  147. 

ture  had  a  great  advantage,  but  as  the  art  of  designing  has 
advanced  the  two  types  have  become  closely  approximated  in 
their  properties. 

In  this  country  the  windings  of  induction  motors  are  gener- 
ally placed  in  open  or  nearly  open  slots,  as  in  the  case  of  the 
motors  shown  in  Plate  VII.  Abroad  the  arrangement  of  wind- 
ings in  holes  as  shown  in  Fig.  145  is  very  common.  Each 
procedure  has  its  advantages.  The  American  practice  renders 
it  very  easy  to  place  the  windings,  and  to  put  a  very  large 
amount  of  copper  upon  the  armature,  for  open  slots  can  be 
made  radially  deep  and  filled  true,  while  holes  unless  rather 
large  can  only  be  trued  by  reaming,  which  implies  a  round 


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ELECTRIC  TRANSMISSION  OF  POWER, 


hole,  unfavorable  if  a  great  amount  of  copper  is  to  be  crowded 
upon  the  armature.  Hence,  with  open  slots  it  is  easier  to  sub- 
divide the  winding  into  many  slots,  thus  reducing  the  armature 
reactance.  On  the  other  hand,  open  slots  are  extremely  unfa- 
vorable as  regards  power  factor,  since  the  iron  surfaces  opposed 
in  armature  and  field  are  very  greatly  reduced,  and  hence  the 
tendency  to  use  extremely  small  clearances  in  order  to  make 
the  best  of  a  bad  matter.  The  European  practice  is  on  the 
whole  better  as  regards  power  factor,  but  does  not  facilitate 
the  construction  of  motors  of  very  low  armature  resistance, 
and  is  considerably  more  difficult  of  proper  execution.    The 


FlO.  148. 

matter  really  hinges  on  the  relative  cost  of  labor  here  and 
abroad.  With  cheap  labor  the  manufacturer  can  afford  to  go 
into  little  refinements  if  it  is  otherwise  worth  while,  but  at 
American  labor-rates  handwork  has  to  be  minimized.  On  the 
whole,  the  American  motors  are  fully  up  to  foreign  standards 
in  general  design,  although  the  tendency  here  has  been  to 
make  a  fetish  of  uniformity  of  speed,  even  at  the  expense  of 
more  important  characteristics. 

In  motors  such  as  those  just  described,  with  distributed 
windings  and  no  sharply  defined  polar  areas,  the  consecutive 
exchange  of  motor  and  transformer  functions  among  the  wind- 
ings is  almost  lost  sight  of  in  the  presence  of  the  very  apparent 
phenomenon  of  resultant  revolving  poles,  but  the  appearance 


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ALTERNATING  CURRENT  MOTORS.  265 

of  the  latter  is  a  necessary  result  of  the  persistence  of  the 
former.  These  induction  motors  are  generally  operated  from 
the  secondary  circuits  of  transformers,  although  the  large 
sizes  (50  HP  and  upward)  are  sometimes  wound  for  use  of 
the  full  primary  voltage  up  to  2,000  volts  or  more. 

An  early  form  of  induction  motor  which  possesses  some 
interestuig  features  is  the  Stanley  machine,  shown  in  Figs.  147, 
148,  149.  The  field  in  Fig.  147  is  composed  of  two  separate 
rings  of  laminated  iron,  each  having  eight  polar  projections. 
These  field  rings  are  assembled  side  by  side  with  the  poles 
** staggered,"  as  shown  in  the  cut.  Each  field  is  energized 
separately,  one  from  each  branch  of  a  two-phase  circuit.  The 
armature,  Fig.  148,  is  composed  of  two  separate  cores  assem- 


Fio.  160. 

bled  side  by  side.  The  secondary  winding,  Fig.  149,  polyodon- 
tal  as  usual,  is  common  to  the  two  cores.  The  transformer 
and  motor  functions  are  here  separated,  for  each  half  of  the 
machine  acts  alternately  as  transformer  and  motor,  each  set  of 
fields  inducing  current  which  serves  for  motor  purposes  in  the 
other  half  of  the  machine.  There  is  no  rotary  field  in  the 
ordinary  sense  of  that  term,  since  there  is  no  physical  resul- 
tant of  the  two  field  magnetizations,  nothing  but  the  alterna- 
tion of  transformer  and  motor  functions  that  is  a  characteristic 
of  all  polyphase  induction  motors. 

These  motors  have  been  generally  used  in  connection  with 
condensers  to  improve  the  power  factor,  and  to  facihtate  this 
practice  have  been  usually  woimd  for  500  volts. 

A  step  further  in  the  direction  of  simplicity,  but  generally 
inferior  to  both  polyphase  and  heterophase  forms,  are  the  true 
monophase  induction  motors.    The  principle  of  these  motors  is 


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256 


ELECTRIC  TRANSMISSION  OF  POWER, 


shown  in  Fig.  150.  Here  there  is  but  one  set  of  poles  energized 
by  the  circuit  A,  while  6,  c,  d,  are  portions  of  the  armature  wind- 
ing, which  may  be  a  simple  squirrel  cage,  or  a  complex  bar 
winding  similar  to  those  used  in  polyphase  motors. 

If  A  be  supplied  with  an  alternating  current,  induced  cur- 
rents will  be  produced  in  the  armature,  out  of  phase  with  the 
field  magnetization  and  symmetrical  with  respect  to  it,  so  that 
no  torque  is  produced. 

If,  however,  we  spin  the  armature  up  to  nearly  synchronous 
speed,   the  armature   currents   will  lag,   from  self-induction, 


Pio.  151. 

behind  the  E.  M.  F.  set  up  by  the  field,  so  that  they  have  an 
angular  displacement  with  respect  to  the  field  at  a  time  when 
the  latter  is  still  active.  There  is,  therefore,  torque  between 
these  two  elements  —  in  the  direction  of  the  initial  rotation. 
The  motor  will  thus  run,  when  once  started,  equally  well  in 
either  direction. 

In  every  motor  there  must  be  not  only  a  field  magnetization 
and  current  in  a  movable  conductor  substantially  in  phase 
with  each  other,  but  there  must  be  a  stable  angular  displace- 
ment between  the  two  in  order  to  ensure  continuous  torque. 
In  continuous  current  motors  this  displacement  is  secured  by 


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ALTERNATING  CURRENT  MOTORS.  257 

the  position  of  the  brushes.  In  polyphase  induction  motors  it 
is  obtained  by  the  space  relation  of  the  sets  of  poles  combined 
with  the  time  relation  of  the  two  or  more  currents. 

In  the  monophase  motor  this  angular  displacement  is  due 
to  the  displacement  of  the  armature  currents  by  inductance. 
Hence,  there  is  a  particular  value  of  the  inductance  correspond- 
ing to  the  best  condition  of  torque,  more  or  less  than  this 
being  especially  injurious  in  this  type  of  motor. 

In  practice  monophase  induction  motors  are  built  in  very 


FlO.  162. 

much  the  same  form  as  polyphase  motors,  and  for  the  same 
reason,  i.e.,  to  make  the  structure  good  as  a  transformer.  In 
fact,  the  same  motor  structures  are  often  used  for  both  types. 
Fig.  151  shows  the  manner  of  winding  a  six-pole  monophase 
primary,  homologous  with  Figs.  141,  142.  A  monophase 
induction  motor  of  120  HP  by  Brown,  Boveri  &  Co.,  is  shown 
in  Fig.  152.  Monophase  induction  motors  are  not  yet  used  to 
any  large  extent  in  this  countr}',  and  abroad  their  use  is  gen- 
erally confined  to  motors  much  smaller  than  the  example 
shown. 

A  moment's  reflection  will  show  that  while  the  supply  of 


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ELECTRIC  TRANSMISSION  OF  POWER. 


energy  to  a  polyphase  motor  is  substantially  continuous,  in 
monophase  motors  it  is  essentially  intermittent,  so  that  the 
latter  give  less  output  for  the  same  structure,  while  the  depen- 
dence of  the  torque  on  the  armature  inductance  generally 
leads  to  low  power  factors. 

Nevertheless,  cases  arise  in  which  it  is  extremely  convenient 
to  use  single  phase  motors.     There  are  still  many  small  light- 


FlO.  163. 

ing  plants  equipped  only  with  single  phase  generators,  the 
extreme  simplicity  of  the  circuits  out-weighing  the"  advan- 
tage of  polyphase  service  in  economy  of  copper  and  ready 
availability  of  motor  service.  For  the  occasional  motors  de- 
sirable on  such  systems  some  form  of  monophase  machine  is 
important.  There  are  also  some  large  lighting  plants  that 
serve  their  territory  both  by  continuous  current  and  in  the 


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ALTERNATING  CURRENT  MOTORS. 


25d 


remoter  districts  by  alternating  current  for  which  such  motors 
are  useful,  and  even  on  polyphase  systems  there  are  isolated 
demands  for  motors  which  can  best  be  filled  by  a  circuit  run 
from  a  single  phase. 

Monophase  motors  are,  too,  rather  more  cheaply  installed 
on  account  of  the  simpler  circuits  and  lower  cost  of  trans- 
formers, so  that  there  is  a  genuine  though  at  present  rather 
limited  demand  for  them.  As  a  result  there  have  been  deter- 
mined and  measurably  successful  efforts  to  produce  practical 
monophase  motors  capable  of  use  at  least  in  small  sizes,  with- 
out the  impairment  of  general  regulation  likely  to  come  from 
low  power  factor  and  large  current  at  starting.  Abroad  the 
ordinary  monophase  type  just  described  is  used,   generally 


100 

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80 

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Fig.  154. 

Started  light  and  taking  up  its  load  with  a  clutch,  but  here  a 
self-starting  motor  is  universally  demanded. 

One  of  the  recent  American  contributions  to  the  list  of 
monophase  motors  is  somewhat  out  of  the  ordinary  in  that  it 
starts  as  an  induction  motor  by  the  aid  of  a  commutator. 
This  is  the  Wagner  motor  shown  in  Fig.  153.  In  its  general 
construction  it  is  a  pure  monophase  motor  with  an  armature 
winding  the  coils  of  which  are  at  one  end  connected  \ivith  a 
commutator.  This  has  bearing  on  it  a  pair  of  brushes  which 
close  upon  themselves  those  armature  coils  which  are  in  such 
angular  relation  with  the  field  magnetization  as  to  give  a 
strong  motor  reaction  with  it.    By  thus  keeping  in  action 


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260 


ELECTRIC  TRANSMISSION  OF  POWER, 


only  coils  giving  an  efficient  torque  in  one  direction,  the 
necessary  directed  torque  at  starting  is  secured,  and  when  the 
motor  reaches  a  predetermined  speed  a  compact  little  centri- 
fugal governor  throws  over  a  short-circuiting  ring,  converting 
the  motor  into  an  ordinary  monophase  induction  motor.  It 
is  possible  to  start  under  load  with  this  device  by  drawing 
rather  heavily  on  the  mains  for  current,  but  in  any  except  the 
smallest  sizes  it  is  better  to  start  light. 

Fig.  155  shows  the  characteristic  curves  of  a  recent  4  HP 
Wagner  motor,  which  gave  a  highly  creditable  performance  for 
a  monophase  motor  of  so  small  size.     It  comes  much  nearer 


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representing  real  commercial  conditions  than  the  curves  of 
Fig.  154,  and,  as  we  shall  presently  see,  does  not  make  a  bad 
showing  as  compared  with  the  polyphase  motors  ordinarily 
found  upon  the  market. 

Although  monophase  motors  as  a  class  start  at  a  great  dis- 
advantage compared  with  polyphase  motors,  they  can  be  made 
to  give  pretty  good  results  at  load  by  extraordinary  care  in 
designing.  Fig.  154  shows  the  curves  obtained  from  a  certain 
Brown  motor  by  Professor  Amo.  The  motor  was  nominally 
of  15  HP,  but  was  evidently  overrated  at  that  load.  Never- 
theless, within  a  certain  range  of  load  the  performance  of  this 


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Fig.  2. 


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ALTERNATING  CURRENT  MOTORS.  261 

motor  compares  well  with  that  of  the  best  polyphase  motors 
of  similar  size.  This  motor  had  an  extremely  small  air  gap, 
and  shows  doubtless  a  record  performance  in  several  respects, 
but  it  proves  that  barring  the  matter  of  starting  it  woidd 
be  possible  to  turn  out  a  pretty  usefid  machine  of  the  mono- 
phase type  if  anybody  desired  it,  although  it  is  certain  that 
at  anything  like  equal  cost  of  construction  the  polyphase 
motor  must  retain  the  advantage.  Fig.  1,  Plate  VIII,  shows 
a  recent  monophase  motor  of  Westinghouse  make.  It  is  in 
appearance  hardly  distinguishable  from  the  polyphase  motors, 
and  its  operative  qualities  are  said  to  be  excellent.  Speed 
regidation,  never  any  too  easy  in  induction  motors,  is  almost 
out  of  the  question  in  the  monophase  form.  Still,  within  its 
limitations  it  has  its  uses. 

A  very  ingenious  flank  movement  has  recently  been  made 
by  the  General  Electric  Co.,  upon  the  monophase  problem  in 
a  monophase  induction  motor  with  condensers.  In  polyphase 
motors  the  usefulness  of  condensers  had  been  shown  by  the 
Stanley  type  previously  mentioned,  and  the  same  device  seems 
specially  usefid  in  overcoming  the  low  power  factor  to  which 
the  monophase  form  is  especially  prone.  Plate  VIII,  Fig.  2, 
shows  the  2  HP  monophase  motor  of  this  type,  moimted  upon 
a  base  which  contains  the  condenser.  Its  weight  is  295  lbs., 
its  nominal  speed  1,800  r.p.m.  at  60^^  and  its  slip  at  full  load 
2.75  per  cent.  Its  fidl  load  eflSciency  is  75  per  cent,  and 
power  factor  92  per  cent.  The  condenser  is  hermetically  sealed 
in  a  tin  case  and  is  connected  not  as  a  shunt  to  the  whole 
field,  but  is  closed  upon  an  independent  phase  winding,  so 
that  the  motor  belongs  rather  to  the  split-phase  class  than 
to  the  strictly  monophase.  The  armature  likewise  is  given  a 
winding  akin  to  that  of  the  ordinary  polyphase  motors  and  is 
provided  with  a  starting  resistance,  in  series  with  the  armature 
windings  at  starting  and  cut  out  automatically  as  the  arma- 
ture nears  speed,  by  a  centrifugal  switch.  An  automatic 
clutch  pulley  is  also  provided  on  these  motors  to  further  facil- 
itate starting  with  moderate  current. 

As  might  be  expected  from  these  features  the  motor  is  singu- 
larly free  from  starting  and  power  factor  difScidties,  and,  at 
the  cost  of  some  complication  to  be  sure,  meets  the  end  for 


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262 


ELECTRIC  TRANSMISSION  OF  POWER. 


which  it  was  designed,  of  furnishing  a  motor  suitable  for  con- 
nection to  single  phase  lighting  circuits  without  material  risk 
of  injiuing  the  regulation. 

Fig.  156  shows  the  characteristic  curves  from  a  5  HP  motor 
of  this  class.  It  will  be  observed  that  the  full  load  power 
factor  is  .95  and  the  real  efficiency  at  the  same  point  .80,  which 
is  certainly  an  excellent  showing.  Obviously  the  power  factor 
is  a  matter  of  proportioning  the  condensers,  and  in  motors 
of  this  class  of  10  HP,  and  upwards  the  power  factor  is  raised  to 
unity,  or  the  ciurent  is  even  made  to  lead  at  certain  loads. 


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Fio.  156. 


These  various  recent  forms  of  monophase  motor  have  come 
into  somewhat  considerable,  though  scattered  use,  mostly  in 
sizes  below  10  HP,  although  now  and  then  motors  of  several 
times  this  power  have  been  installed.  When  judiciously  in- 
stalled they  can  undoubtedly  be  made  to  give  good  service. 

There  has  also  very  recently  been  introduced  a  most  inter- 
esting type  of  single  phase  alternating  ciurent  motor  derived 
from  and  closely  resembling  in  its  properties  the  ordinary 
direct  current  series  motor.  It  is  in  fact  a  series  motor  spe- 
cialized for  alternating  current  working. 

The  direction  of  rotation  of  a  series  motor  depends  entirely 


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ALTERNATING  CURRENT  MOTORS.  268 

on  the  direction  of  the  magnetizations  produced  by  the  field 
and  armature  respectively.  Consequently  it  does  not  change 
if  the  direction  of  the  current  be  reversed  at  the  motor  termi- 
nals, but  only  if  it  be  reversed  as  between  field  and  armature. 
Hence,  such  a  motor  would  run  even  if  supplied  at  its  terminals 
with  alternating  current,  provided  enough  such  current  coidd 
be  forced  through  in  spite  of  the  high  inductance  of  the  ma- 
chine. The  first  step  toward  this  end  would  evidently  be  to 
reduce  the  frequency,  but  evidently  even  at  low  frequency  the 
losses  from  eddy  currents  in  the  solid  iron  would  be  serious, 
and  the  next  obvious  step  is  to  construct  the  field  as  well  as 
the  armature  of  iron  laminated  like  a  transformer  core.  In 
fact  it  has  been  known  for  a  long  time  that  a  series-wound 
motor  with  a  laminated  field  would  operate  after  a  fashion 
when  fed  with  low  frequency  alternating  currents,  say  at  8  or 
10  periods  per  second.  The  recent  work  has  been  in  the  direc- 
tion of  so  specializing  this  machine  as  to  keep  down  the  in- 
ductance and  to  reduce  the  sparking  to  reasonable  limits  when 
operating  at  the  lower  commercial  frequencies. 

The  chief  electrical  feature  of  the  a.c.  series  motor  is  that  its 
total  counter  E.  M.  F.  is  the  geometrical  sum  of  the  E.  M.  F.'s 
induced  by  the  motion  of  the  armature  conductors  and  those 
due  to  reactance  in  the  armature  and  field  respectively.  Now 
the  apparent  watts  supplied  are  measured  by  the  product  C  E^y 
where  E^  is  the  impressed  E.  M.  F.  while  the  useful  energy  is 
determined  by  E,  the  motor  E.  M.  F.  as  in  any  other  motor. 
Hence,  in  order  that  an  a.c.  series  motor  should  have  a  good 
power  factor  and  apparent  efficiency,  it  is  necessary  to  make 
E  large  compared  with  the  reactances  of  armature  and  field. 
To  do  this  the  niunber  and  the  speed  of  the  armature  wires 
may  be  increased  on  the  one  hand  and  the  reactances  kept 
down  upon  the  other.  The  first  condition  points  to  a  motor 
having  a  relatively  simple  field  and  a  very  powerful  high  speed 
armature,  while  the  second  condition  calls  for  low  frequency 
and  very  careful  designing  against  reactance. 

To  reduce  the  field  reactance  the  turns  on  the  field  must 
be  kept  low,  since  one  cannot  reduce  the  effective  field  magnet- 
ization without  reducing  that  in  the  armature  also,  and  to 
maintain  the  field  with  few  turns,  requires  a  small  air  gap  of 


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264  ELECTRIC  TRANSMISSION  OF  POWER. 

large  area.  Since  one  cannot  reduce  the  air  gap  beyond  a 
certain  point  without  mechanical  difficulties,  and  cannot 
increase  its  area  much  without  increasing  the  general  dimen- 
sions of  the  motor,  the  saving  of  reactance  in  the  field  is  neces- 
sarily rather  limited. 

One  can,  however,  considerably  reduce  the  armature  reac- 
tance by  winding  a  neutralizing  coil  so  as  to  surrovmd  the 
revolving  armature  in  a  plane  approximately  perpendicular 
to  the  line  joining  the  brushes.  This  is  known  as  a  compen- 
sating coil  and  the  motor  fitted  with  it  as  a  compensated 
series  motor.  Fig.  157  shows  this  arrangement  in  diagram. 
The  result  is  to  very  greatly  diminish  the  net  armature  reactance 

so  that  the  power  factor  may  be 
carried  to  .90  or  even  more.  Here 
A  is  the  armature,  F  the  field,  and 
C  the  compensating  coil. 

The  recently  introduced  single- 
phase  motors  for  electric  traction 
generally  belong  to  this  type,  and 
in  certain  cases  these  machines 
may  be  useful  for  variable  speed 
work  on  commercial  circuits,  for 
they  behave  imder  supply  at  varied 
voltage  quite  like  d.c.  series  motors 
and  indeed  can  be  worked  on  d.c.  circuits.  Fig.  158  shows  a 
Westinghouse  single-phase  railway  motor  with  the  armature 
removed,  showing  the  field  coils  and  the  compensating  coils. 
A  modification  of  the  same  idea  is  shown  in  Fig.  159,  where 
the  compensating  coil  is  short  circuited,  the  motor  being  other- 
wise arranged  as  before.  Still  another  commutating  type  of 
a.c.  motor  is  that  shown  diagrammatic  ally  in  Fig.  160  in  which 
there  are  field  and  compensating  coils  in  series,  but  the  arma- 
ture is  short  circuited  upon  itself.  This  is  substantially  like 
Prof.  Elihu  Thomson's  repulsion  motor  in  the  original  form 
of  which,  however,  the  coils  F  and  C  were  replaced  by  a  small 
resultant  coil  in  an  intermediate  position  with'  respect  to  the 
brush  line.  This  motor  too  has  been  developed  for  railway 
work,  and  has  much  the  same  properties  as  the  regular  series 
compensated  type. 


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ALTERNATING  CURRENT  MOTORS. 


265 


All  these  alternating  motors  have  good  power  factors  when 
working  near  their  normal  speeds,  often  rising  to   .90  and 


Fio.  168. 


more,  but  their  efficiency  is  generally  materially  less  than  that 
of  d.c.  motors,  or  polyphase  induction  motors  of  similar  out- 
put.    The  losses  from  the  more  complex  windings,  from  eddy 


Fio.  169. 


Fio.  160. 


currents  and  from  hysteresis  are  enough  to  cut  down  efficiency 
generally  5  to  10  per  cent,  more  often  near  the  latter  figure. 

By  careful  design  of  the  commutation  sparking  can  be  kept 
within  reasonable  bounds,  at  least  within  a  moderate  range 


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266 


ELECTRIC  TRANSMISSION  OF  POWER. 


of  speedy  although  the  conditions  for  sparkless  operations  can 
never  be  as  favorable  as  in  d.c.  motors. 

Several  other  modifications  of  the  commutating  a.c.  motor 
have  been  devised,  but  they  all  depend  on  principles  similar 
to  those  already  mentioned,  and  may  be  expected  to  perform 
in  about  the  same  way.  None  of  them  can  reasonably  be 
expected  to  do  materially  better  than  the  series  compensated 


aoaoaoio    6000    70    so    soiooiiouo  a.p] 

Fio.  161. 


type  first  described.  They  are  one  and  all  intended  for  low 
frequency  work  generally  25  -^  as  a  maximum.  The  higher 
the  frequency  the  harder  to  build  a  good  commutating  motor. 
Hence,  whatever  place  such  motors  may  find  in  traction,  they 
will  probably  be  of  rather  limited  applicability  on  ordinary 
commercial  circuits  of  the  present  usual  frequency  of  60^, 
although  they  may  be  occasionally  useful. 

The  practical  properties  of  good  modem  induction  motors 
are  strikingly  similar  to  those  of  shunt-wound  or  separately 
excited  continuous  current  motora. 

For  the  same  output,  the  induction  motor  generally  has  the 
advantage  in  weight,  owing  to  the  fine  quality  of  iron  which  has 


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ALTERNATING  CURRENT  MOTORS, 


267 


to  be  employed,  but  its  laminated  structure  and  rather  com- 
plicated primary  winding  make  it  fully  as  expensive  to  build,  in 
spite  of  the  absence  of  a  commutator. 

In  point  of  commercial  eflficiency  there  is  but  little  differ- 
ence. It  is  not  difficult  to  build  an  induction  motor  which  is 
fully  up  to  the  average  efficiency  of  other  motors  of  similar 
output  and  speed.  And  what  is  of  greater  importance,  the 
question  of  sparking  being  eliminated,  the  point  of  maximum 
efficiency  can  quite  easily  be  brought  somewhere  near  the  aver- 


OUTPUT-BRAKE  H.P.. 
FlO.  162. 


age  load.  It  must  be  remembered  that  here,  as  elsewhere, 
the  last  few  per  cent  of  efficiency  are  somewhat  costly,  and  not 
always  found  in  the  rank  and  file  of  commercial  machines. 

The  weak  point  of  commercial  induction  motors  is  apt  to  be 
the  power  factor.  Of  course  low  power  factor  means  demand 
for  current  quite  out  of  proportion  to  the  output,  and  hence 
greater  loss  in  the  lines  and  greater  station  capacity.  In 
addition,  a  heavy  lagging  current  makes  regulation  of  voltage 
on  the  system  anything  but  easy. 


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268 


ELECTRIC  TRANSMISSION  OF  POWER. 


Now,  it  is  perfectly  feasible  to  build  induction  motors  with 
power  factors  so  high  as  to  avoid  these  practical  difficidties 
almost  entirely.  But  this  result  is  somewhat  expensive, 
whether  reached  by  finesse  in  design,  or  by  the  addition  of  con- 
densers, and  it  is  therefore  not  always  attamed. 

Slow  speed  induction  motors,  large  and  small,  are  subject 
to  bad  power  factors,  and  so  in  fact  are  all  induction  motors 
having  many  poles.  The  best  results,  however,  are  very  good 
indeed.     A  power  factor  of  .9  or  thereabouts  at  normal  load 


t    10    li    u    i» — A    Hb   m    k    m 

MECHANICAL  HORSE  POWER 
FlO.  163. 


is  quite  unobjectionable  in  practice,  and  this  figure  can  be 
reached  or  closely  approximated  by  careful  design. 

In  point  of  efficiency  there  is  little  difficulty  in  reaching 
satisfactory  figures.  The  actual  properties  of  polyphase  induc- 
tion motors  can  be  best  appreciated  by  the  examination  of 
their  characteristic  curves,  showing  the  variations  of  efficiency, 
power  factor,  and  speed  under  varying  loads.  Fig.  161  shows 
these  curves  for  a  75  HP  three-phase  motor  built  by  the  General 
Electric  Company.  It  is  a  60^^  motor,  intended  for  severe  ser- 
vice, and  hence  is  arranged  to  carry  considerable  overload  at  a 


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ALTERNATING  CURRENT  MOTORS. 


269 


good  efficiency.  The  fall  in  speed  from  no  load  to  fidl  load  is 
but  3  per  cent,  and  the  starting  torque  is  80  per  cent  greater 
than  full  running  torque,  with  an  expenditure  of  current  closely 
proportional  to  the  torque.  The  commercial  efficiency  reaches 
91.1  per  cent,  and  the  power  factor  84.3  per  cent,  which  is  not 
bad  for  so  large  a  motor  intended  for  considerable  overloads. 
Fig.  162  shows  the  characteristics  of  a  Westinghouse  two- 
phase  induction  motor  of  50  HP  for  25-^.  Its  properties,  as 
might  be  expected  of  a  well-designed  motor  for  so  low  a  fre- 


iooa  ■KMtfuJbofr 


FlO.  161 


quency,  are  admirable,  particidarly  the  great  efficiency  at 
small  loads. 

Fig.  163  shows  the  properties  of  a  polyphase  motor  of  20 
HP  at  130^,  used  with  Stanley  condensers  to  keep  down  the 
results  of  the  inductance  encountered  at  so  high  a  frequency. 
The  effect  of  this  device,  particularly  at  moderate  loads,  is 
very  striking  indeed.  Without  condensers  one  could  not 
obtain  such  a  power  factor  even  at  full  load.  While  the  con- 
denser does  not  perfectly  compensate  for  inductance,  it  does 
so  sufficiently  well  for  all  practical  purposes.  In  other  prop- 
erties the  motor  is  not  so  especially  remarkable. 

These  curves  are  from  the  manufacturers'  tests,  and  the 


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270 


ELECTRIC  TRANSMISSION  OF  POWER. 


author  believes  them  to  be  entirely  trustworthy,  although  they 
probably  represent  good  results.  Better  curves  than  these  are 
occasionally  obtained,  generally  for  some  individual  reason. 
Now  and  then  a  "freak"  motor  is  produced,  with  enormously 
high  efficiency  or  power  factor,  like  a  certain  5  HP  three-phase 
motor  designed  and  tested  by  the  author,  which  gave  at  full 
load  a  power  factor  of  .94. 

On  the  other  hand,  it  is  unfortunately  true  that  many  com- 
mercial induction  motors  are  not  as  good  in  point  of  efficiency 
and  power  factor  as  they  ought  to  be.  A  series  of  tests  of 
induction  motors  under  the  direction  of  Professor  D.  C.  Jack- 
son was  published  a  few  years  since,  which  gives  data  so  instruc- 
tive and  impartial  as  to  be  well  worth  reproduction  here.  The 
motors  tested  were,  except  for  a  10  HP  Westinghouse  two- 
phase,  all  of  5  HP  nominal  capacity,  and  by  the  following 
makers:  Westinghouse,  Fort  Wayne  Electric  Corporation  (syn- 
chronous self-starting  monophase),  Stanley,  AUegemeine  Elec- 
tricitats  Gesellschaft,  General  p]lectric  Company.  In  addition, 
results  of  tests  on  Oerlikon  and  Brown  motors  are  inclu4ed  in 
the  results.  Fig.  164  shows  the  efficiency  curves  and  regu- 
lation of  the  several  machines,  and  the  table  gives  a  general 
view  of  their  respective  properties. 

COMPARATIVE    QUALITIES   OF  INDUCTION    MOTORS. 


1 
2 
3 

4 

6 

610 

7 

8 


1 

L 

I 

p3 


5 
6 

6.26 
6 

666 
10 
4.6 
6 


Torque,  In  Per 

Cent  of  Full 

Load. 


140 

100 

104 

186.5 

171 

147 

163 


131 

46 

90 
136 
138.6 
142 
232 


^  -L 


3 


Efficiencies,  Per 
Cent. 


77.8  65.2  70.6  77 

83.8  54.5  72.5  80.6  83.8 


Power  Factors,  Per 
Cent. 


9.7,79 
7     79.6 
3.7188 
4.5182 
3.7:79.161 
4.683. 8'66.6 
I I 


171.9 
62.2  77.2 
75.2  85 


67 


78.4 


78.fl|77 

79.5I77.8 

87.788 


76.6  76.4  13 
67.5  10. < 

Si     174 
82. 6,16. J 
•8.5    6.1 


37.6 
5:25.5 

81.6 
5'44.7 

8.27.6 
81.9  81.li80    II6     40.4 
73.8  78.4,79. 1,88.4  22.2  51 .3 
62 


80.283.882      89 


62.3 

67.3 

3  83.7 

5,73.7 

659.8 

480.3 
6|81.7 


71.6 

64 

84 

80.3 

70.3 

73.4 

85.2 

87.2 


Note.  —  6, 7,  and  8  were  not  run  up  to  maximum  load,  on  test. 


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ALTERNATING  CURRENT  MOTORS.  271 

Ix)oking  over  these  results,  Nos.  3,  5,  and  8  are  decidedly 
the  best  of  the  lot.  Of  these,  No.  3  is  possessed  of  a  fairly  high 
and  very  uniform  power  factor,  but  rather  moderate  efficiency. 
It  starts  well,  and  ^ith  a  moderate  current  has  sufficient 
margin  of  capacity  for  all  ordinary  work,  but  its  speed  falls 
considerably  under  load.  No.  5  has  extraordinary  efficiency  at 
all  loads,  starts  admirably,  and  can  carry  a  tremendous  over- 
load —  more  than  double  its  rated  capacity.  Moreover,  it 
regulates  very  closely.  The  power  factor,  however,  is  so  bad 
as  to  be  a  curiosity,  having  apparently  been  sacrificed  to 
obtain  great  maximum  output,  which  is  for  many  purposes  use- 
less. No.  8  is  a  far  better  all-round  machine  than  any  of  the 
others,  has  a  good  maximum  efficiency  at  a  little  below  full 
load,  and  an  excellent  power  factor.  Professor  Jackson  notes 
that  since,  at  an  output  of  3J  HP,  No.  3  has  an  efficiency  of 
75.5,  and  a  power  factor  of  83 J,  while  No.  5  shows  respec- 
tively 85  and  59,  the  station  capacity  for  the  latter  must  be  con- 
siderably greater  than  for  the  former.  That  is,  the  apparent 
efficiency  of  Nq.  3,  which  determines  the  necessary  station 
capacity,  is  64  per  cent,  while  that  of  No.  5  is  50  per  cent. 
Hence,  to  supply  one  brake  HP  with  No.  5  motors,  there  must 
be  a  station  capacity  of  2  HP,  while  with  No.  3  motors  1.56  HP 
is  sufficient.  But  with  No.  8  the  efficiency  is  about  .83,  and  the 
power  factor  about  .80,  giving  an  apparent  efficiency  of  .66, 
which  is  better  than  either  No.  3  or  No.  5.  Motors  like 
No.  3  are  excellent  for  the  power  station,  but  hard  on  the 
customer,  while  No.  5  is  admirable  for  the  customer,  but  bad 
for  the  station.     No.  8  is  fair  to  both  parties. 

Most  of  the  motors  shown  start  quite  well  enough  for  ordi- 
nary purposes.  Neither  heavy  starting  torque  nor  ability  to 
carry  large  overloads  is  needed  in  ordinary  motor  work. 
Large  torque  per  ampere  is,  however,  desirable.  It  is  best 
secured  by  using  at  starting  a  non-inductive  resistance  in  the 
secondary  circuit  as  found  in  many  existing  motors.  The 
actual  effect  of  this  resistance  is  as  follows:  It  reduces  the 
current  drawn  from  the  mains  so  that  the  motor  will  not 
seriously  disturb  the  voltage  on  the  lines  at  starting;  by  dimin- 
ishing the  current  flowing  in  the  armature  it  limits  the  arma- 
tiu-e  reaction  so  that  it  may  not  beat  back  the  field  so  as  to 


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272  ELECTRIC  TRANSMISSION  OF  POWER. 

interfere  with  proper  starting,  nor  distort  it  so  as  to  produce 
dead  points;  and,  finally,  it  largely  increases  the  torque  per 
ampere,  which  greatly  aids  in  starting  under  load. 

The  function  first  mentioned  is  very  important  where  lights 
and  motors  are  to  be  operated,  since  if  a  motor  is  capable  of 
starting  imder  heavy  load  it  is  likely  to  take  at  starting  a 
pretty  large  current,  which  may  pull  down  the  voltage  in  the 
neighborhood  merely  in  virtue  of  ohmic  drop.  Besides,  the 
power  factor  of  an  induction  motor  at  starting  is  only  about 
.7,  so  that  the  heavy  current  lags  severely  and  still  further 
interferes  with  proper  regulation. 

The  heavy  lagging  current  set  up  in  the  armature  is  likely 
to  distort  the  field  seriously,  sometimes  so  much  as  to  block 
the  starting  of  the  motor,  sometimes  merely  producing  dead 
points,  i.e.,  points  of  no  torque,  or  greatly  weakening  the 
torque  in  certain  positions  of  the  armature.  The  introduc- 
tion of  resistance  in  the  secondary  circuit  both  diminishes  the 
current  and  its  angle  of  lag,  and  thus  keeps  down  the  arma- 
ture reaction.  In  some  motors  the  reluctance  of  the  magnetic 
circuits  is  sensibly  the  same  in  all  angular  positions  of  the 
armature,  so  that  there  are  no  points  of  noticeably  weak 
torque  either  with  or  without  a  starting  resistance.  But 
some  motors  otherwise  excellent  have  sufficient  variations  of 
reluctance  to  produce  bad  dead  points  when  the  armature 
reactance  is  severe,  while  these  nearly  or  quite  disappear  by 
adding  resistance  in  the  secondary  circuits. 

The  use  of  resistance  in  the  secondary  at  starting  obviously 
throws  forward  the  phase  of  the  secondary  current  so  that  it 
is  in  better  relation  to  the  field  magnetization,  and  hence 
although  the  numerical  value  of  the  current  is  reduced,  its 
effective  component  is  increased.  The  considerations  which 
affect  the  relations  between  torque  and  current  in  the  arma- 
tures of  induction  motors  are  in  reality  quite  simple.  The 
absolute  value  of  the  current,  other  things  being  equal,  is 
determined  by  the  armature  impedance,  and  is  the  same  for 
the  same  impedance  whatever  the  relation  between  the  reac- 
tance and  resistance  comiponents  of  that  impedance.  The 
ratio  between  these  components,  however,  determines  the 
phase  angle  of  the  armature  current,  so  that  for  a  given  value 


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ALTERNATING  CURRENT  MOTORS,  273 

of  the  cun*ent  the  torque  depends  on  the  ratio  between  resis- 
tance and  reactance  in  the  armature.  x 

By  lessening  either  the  resistance  or  reactance  of  the  arma- 
ture a  motor  is  obtained  in  which  a  very  large  ciu'rent  flows 
at  starting,  but  reducing  the  impedance  by  cutting  down 
reactance  gives  the  resulting  current  a  better  phase  angle 
than  that  obtained  by  reducing  resistance  alone.  For  a  given 
motor  the  maximum  torque  is  obtained  when  the  ratio  of 
resistance  and  reactance  is  unity,  i.e.,  when 

7  =  R. 

Now,  one  can  cut  down  the  resistance  by  increasing  the 
allowance  of  armature  copper,  and  can  diminish  the  reactance 
by  subdividing  the  winding  so  that  there  shall  be  many  slots 
in  the  armature,  and  the  minimum  possible  number  of  turns 
per  slotr  Also  the  better  the  mutual  induction  between  field 
and  armature  the  less  the  reactance  of  either  member  is  likely 
to  be,  so  that  by  close  attenticm  to  design  it  is  possible  greatly 
to  reduce  the  armature  reactance.  In  commercial  motors  the 
relation  between  resistance  and  reactance  in  the  armature  is 
generally  from 

/  =  3  i2  to  /  =  10  fi. 

Hence,  when  large  torque  per  ampere  is  desired  the  simplest 
thing  to  do  is  to  insert  non-inductive  resistance  in  the  secon- 
dary, and  when 

/  =  22 

the  given  motor  ^vill  be  at  its  best  with  respect  to  starting 
torque.     With 

7  =  ft 

the  maximum  torque  will  be  obtained  when  both  are  as  small  as 
possible.  Hence,  if  very  great  starting  torque  is  desired,  the 
motor  should  be  designed  witH  very  low  armature  resistance 
and  reactance. 

The  slip  of  the  motor  below  synchronous  speed  depends 
upon  the  armature  resistance  in  induction  motors,  just  as  in 
continuous  current  motors  the  slip  below  the  speed  at  which 
the  armature  would  give  the  impressed  E.  M.  F.  is  determined 


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274  ELECTRIC  TRANSMISSION  OF  POWER. 

by  armature  resistance.  In  each  case  the  slip  measures  the 
percentage  o#^ energy  lost  in  the  armature,  so  that  if  an  induc- 
tion motor,  for  example,  runs  loaded  at  5  per  cent  slip  the  loss 
of  efficiency  in  the  armature  is  5  per  cent. 

Commercial  induction  motors  vary  widely  in  slip  —  from  as 
little  as  1  per  cent  to  8  or  10  per  cent,  according  to  design. 
It  must  not  for  a  moment  be  supposed,  however,  that  small 
slip  implies  high  efficiency  of  the  motor.  One  can  put,  in 
designing  a  motor,  most  of  the  loss  into  the  armature  or  into 
the  field,  as  one  pleases,  and  it  is  pretty  safe  to  say  that  if  there 
is  remarkably  little  in  the  armature  there  will  be  an  unusual 
amount  in  the  field,  unless  cost  is  utterly  disregarded.  Prob- 
ably the  best  all-around  results  can  be  obtained  by  dividing 
the  permissable  loss  nearly  equally  between  armatiu^  and 
field. 

There  is  a  very  simple  relation  between  the  static  and  run- 
ning torques  of  an  induction  motor,  the  static  and  running 
currents,  and  the  slip,  as  follows: 

T        ,,C» 

—  —  is • 

T.         C.> 

In  this  equation  T,  is  the  static  torque,  C,  the  static  current, 

Tg  and  C,  torque  and  current  of  the  slip  S,  and  S  that  slip 

expressed  as  a  percentage.     As  an  example  of  the  application 

of  this  formula,  suppose  the  full  load  current  of  a  certain 

motor  is  60  amperes  per  phase,  the  current  with  the  armature 

at  rest  400  amperes,  and  the  sUp  at  full  load  is  5  per  cent. 

Then 

T         ^^       160,000      ^^^ 

—  =   .05  X =2.22, 

T,  3600 

I.e.,  the  static  torque  will  be  2.22  times  the  full  load  running 
torque.  Of  course,  if  a  motor  is  to  have  a  powerful  starting 
torque  it  must  take  a  pretty  heavy  current,  but  the  extra 
resistance  at  starting  helps  very  materially  in  keeping  the 
current  within  bounds.  An  adjustable  secondary  resistance 
makes  it  easy  to  bring  to  speed  any  load  that  the  motor  will 
carry  continuously,  without  demanding  excessive  current. 
As  to  overload,  an  ability  to  carry  25  per  cent  more  than 


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ALTBRNATtNO  CURRENT  MOTORS. 


276 


the  rated  capacity  is  ample,  save  in  rare  cases,  and  greater 
margin  than  this  usually  means  some  sacrifice  in  efficiency 
or  power  factor  at  normal  loads.  For  most  work  an  effi- 
ciency curve  Uke  that  of  No.  8  is  preferable  to  one  like  that 
of  No.  5.  When  great  margin  of  capacity  is  needed,  it  is  best 
to  use  a  motor  deliberately  adjusted  to  such  use,  and  not  to 
expect  it  of  a  motor  properly  designed  for  ordinary  service. 
The  speed  of  induction  motors  is  best  regulated  by  inserting 


19 

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11 

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Fio.  165. 

a  non-inductive  resistance  in  the  secondary  circuit.  Under 
these  circumstances  the  motor  can  be  made  to  nm  at  con- 
stant torque  over  a  very  wide  range  of  speeds  by  varjdng  the 
resistance,  just  as  one  would  regulate  a  street-car  motor. 
Fig.  165  shows  the  variation  in  speed,  current,  and  power 
factor  in  a  15  HP  three-phase  motor  fitted  with  rheostatic 
control.  The  speed  was  varied  at  constant  torque  from  about 
1,400  r.  p.  m.  down  to  150  r  .p.  m.  Curve  B  shows  the  variation 
of  the  power  factor,  in  this  case  high  at  all  speeds,  and  curve 


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276  ELECTRIC  TRANSMISSION  OF  POWER, 

C  shows  the  slight  variation  iji  input.  Operated  in  this  way, 
the  motor  behaved  almost  exactly  like  a  series-w'ound  direct 
current  motor  with  rheostatic  control.  Such  a  rheostat  is 
used  in  operating  hoists  and  the  like  with  induction  motors. 
Regulation  by  varying  the  primary  voltage  is  highly  imsatis- 
factory,  since  the  torque  /alls  off  nearly  in  proportion  to  the 
square  of  the  voltage,  so  that  at  low  speeds  the  output  is 
enormously  reduced.  Regulation  by  any  method  involving 
resistance  is  of  course  inefficient,  not  materially  more  so,  how- 
ever, than  in  the  case  of  continuous  current  motors.  It  should 
be  understood  that  all  these  remarks  concerning  torque, 
regulation,  and  the  like  apply  to  polyphase  induction  motors 
and  do  not  hold  true  in  general  of  monophase  motors. 

The  w^eak  point  of  induction  motor  practice  is  in  the  heavy 
inductance  likely  to  be  encountered  imless  motors  with  first- 
class  power  factors  are  used.  It  is  depressing  to  find  the 
current  capacity  of  your  generator  exhausted  long  before  it 
has  reached  its  rated  output  in  kilowatts,  and  if  the  motor 
service  is  part  of  a  general  system,  the  effect  of  a  bad  power 
factor  on  regulation  is  disastrous. 

With  generators  of  moderate  inductance  and  good  motors, 
general  distribution  by  polyphase  currents  gives  admirable 
results.  The  station  manager  should  see  to  it  that  his  motors 
are  not  of  excessive  size  for  their  work,  and  are  good  in  the 
matter  of  power  factor.  A  few  motors  for  very  variable  loads 
can  be  handled  readily  enough,  but  no  motor  with  a  bad 
power  factor  should  be  tolerated  simply  because  it  is  cheap. 
Power  factors  of  at  least  .85  at  full  load,  and  .80  at  two-thirds 
load,  are  quite  obtainable  except  in  case  of  some  special  motors, 
and  should  be  insisted  upon  rigorously. 

One  polyphase  station  operating  more  than  fifty  induction 
motors  showed,  when  tested  by  the  author,  about  .65  as  aver- 
age power  factor  when  carrying  all  the  motors.  Rigorous 
inspection  of  the  motors  installed  would  have  raised  this 
figure  to  .75,  although  the  existing  power  factor  actually  gave 
no  trouble,  there  being  ample  generator  capacity. 

So  much  for  polyphase  induction  motors.  Monophase 
motors  generally  fail  to  give  so  uniformly  good  results.  Oc- 
casional extraordinary  results  have  been  reported  from  the 


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ALTERNATING  CURRENT  MOTORS  277 

latter,  but  in  the  author's  opinion  they  concern  motors  which 
belong  to  the  "freak  "  class  alluded  to,  and  cannot  be  expected 
in  commercial  practice.  Monophase  motors  are  usually  weak 
in  power  factor  save  at  certain  loads,  and  start  badly,  most 
of  those  in  use  abroad  being  started  without  load.  Even  so, 
the  starting  current  is  large,  as  may  be  safely  concluded  from 
the  discreet  silence  preserved  on  this  topic  in  all  descriptions 
of  monophase  motor  installations.  In  general  power  trans- 
mission work  the  incandescent  lamp  and  the  induction  motor 
are  the  chief  factors.  Sjmchronous  motors  are  valuable  in 
their  proper  place,  and  arc  lighting  and  continuous  current 
work  are  sometimes  relatively  important.  The  alternating 
current  systems  are  now  far  enough  developed  to  be  entirely 
workable  and  trustworthy  for  incandescents  and  motors. 
The  alternating  arc  lamp  is,  however,  not  quite  in  condition 
to  replace  the  continuous  current  arcs  for  all  purposes  and 
under  all  circumstances,  and  for  work  specially  suited  to  con- 
tinuous cmrents  reliance  has  at  present  to  be  placed  in  various 
current  reorganizing  devices,  which  are,  so  far,  of  rather 
indeterminate  ultimate  value.  Whether  they  are  to  have  a 
large  permanent  place  in  the  art,  or  whether  their  sphere  will 
gradually  be  much  contracted,  is  uncertain.  At  all  events 
it  is  sufficiently  clear  that  the  main  body  of  power  transmission 
will  have  to  depend  on  alternating  currents,  at  least  for  a  long 
while  to  come. 

Even  if  continuous  current  should  be  obtained  somewhat 
directly  from  coal  in  the  near  or  far  future,  the  result  would  be 
not  to  increase  power  transmission  by  continuous  currents, 
but  to  render  the  transportation  of  coal  by  far  the  cheapest 
method  of  transmitting  energy. 

The  relative  importance  of  polyphase,  heterophase,  and 
monophase  systems  is  a  question  often  raised.  The  present 
indications  are  that  the  polyphase  systems,  in  virtue  of  in- 
creased output  of  generators,  possible  economy  in  copper  and 
general  convenience,  have  come  to  stay.  The  monophase 
motor  problem  has  not  yet  been  satisfactorily  solved  in  any 
general  way,  and  until  it  has  been  solved,  the  monophase  sys- 
tem must  remain  subordinate,  like  the  heterophase  systems, 
which  are  special  rather  than  general  in  their  appUcability. 


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278  ELECTRIC  TRANSMISSION  OF  POWER, 

The  much  mooted  question  of  frequency  will  be  referred  to  in 
its  different  bearings  in  connection  with  other  topics.  The 
frequencies  once  common,  120^  to  135^,  are  rapidly  passing 
out  of  use  for  all  important  work.  They  are  inconveniently 
high  for  long  lines  by  reason  of  inductance,  are  troublesome 
for  large  units,  lead  to  high  inductance  in  the  system,  and 
have  for  their  only  compensating  advantage,  lessened  cost  of 
transformers.  Both  here  and  abroad  lower  frequencies  have 
come  into  use.  In  this  country  60^  seems  to  be  the  favorite 
frequency,  except  for  work  with  rotary  converters,  when  25-w 
to  35-^  is  usual.  Both  these  last  are  too  low  for  general 
practice,  since  the  cost  of  transformers  is  greatly  increased; 
the  former  is  unsuitable  for  incandescent  service,  miless  with 
extremely  low  voltage  lamps,  and  both  are  unsuitable  for 
alternating  arcs.  It  is  now  pretty  generally  recognized  for 
the  above  reason,  that  the  adoption  of  so  low  a  frequency  as 
25-w  in  the  great  Niagara  plant  was  an  error  of  judgment, 
perhaps  brought  about  by  an  overestimate  of  the  importance 
of  rotary  converters  in  general  distribution.  The  only  appa- 
ratus which  at  present  demands  low  frequency  is  the  single- 
phase  commutating  motor.  Should  it  come  into  great  use 
plants  of  25*^  or  less  may  be  necessary,  but  for  general  dis- 
tribution it  is  always  preferable  to  keep  the  frequency  high 
enough  for  incandescent  lamps,  which  are  the  most  profitable 
kind  of  load. 

On  the  other  hand,  abroad  a  compromise  frequency  of  40^ 
to  50^  is  in  general  use.  In  the  author's  opinion  there  are 
very  few  cases  in  which  lower  frequencies  than  these  are 
desirable,  and  none  in  which  less  than  30--^  should  be  toler- 
ated for  general  distribution  work;  50^  or  60^  meets  general 
requirements  admirably,  and  only  in  rare  cases  is  the  use  of 
rotary  converters  of  sufficiently  commanding  importance  to 
call  for  a  lower  frequency. 

In  connection  with  this  topic  we  may  consider  a  verbose 
controversy  which  has  raged  of  late,  respecting  the  advantages 
of  certain  irregular  forms  of  alternating  current  waves  vs,  a 
true  sine  wave.  The  facts  in  a  nutshell  are  as  follows:  Cer- 
tain complex  current  waves,  whose  irregularity  is  due  to  the 
presence  of  harmonics  of  higher  frequency,  have  been  found 


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ALTERNATING  CURRENT  MOTORS.  279 

to  give  slightly  better  efficiency  in  transformers  than  sine 
waves  of  the  same  nominal  frequency.  Such  waves,  however, 
do  not  hold  their  form  under  varying  conditions  of  load,  and 
by  reason  of  their  harmonics  of  higher  frequency  raise  the 
inductance  of  the  line  and  apparatus,  increase  the  probability 
of  resonance  on  the  line,  hamper  all  attempts  to  balance  the 
inductance  of  the  system  by  condensers  or  synchronous  motors, 
and  finally  sometimes  interfere  with  the  proper  performance  of 
induction  motors.  The  use  of  such  wave  forms,  then,  is  likely 
to  lead  to  very  embarrassing  complications  in  a  power  trans- 
mission system,  and  their  sole  advantage  is  far  better  secured 
by  using  a  sine  wave  of  slightly  increased  frequency,  than  by 
interpolating  a  set  of  worse  than  useless  harmonics. 

It  is  needless  to  say  that  all  cases  of  power  transmission 
cannot  be  treated  alike  —  there  is  no  system  that  will  meet  all 
conditions  in  the  best  possible  manner.  The  best  results  will 
be  obtained  by  treating,  in  the  preliminary  investigation, 
each  problem  as  an  imique  and  independent  case  of  power 
transmission,  and  afterward  boiling  down  the  conclusions  to 
meet  practical  conditions.  Avoid,  when  you  can,  apparatus 
of  peculiar  sizes  and  speeds  —  remember  that  you  are  after 
results,  not  electrical  curios.  See  to  it  that  what  is  done  is 
done  thoroughly,  and  for  general  guiding  principles  keep  your 
voltage  up  and  your  inductance  down,  and  watch  the  line. 


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CHAPTER  VII. 

CURRENT  REORQANIZERS. 

Whatever  method  may  be  employed  for  the  transmission 
of  power  in  any  given  case,  it  will  often  be  found  that  the 
current  delivered  at  the  receiving  station  is  not  of  the  charac- 
ter needed.  Sometimes  in  transmissions  for  special  purposes 
no  difficulty  ^vill  be  met,  but  frequently,  especially  in  the 
transmission  of  power  for  general  distribution,  both  continu- 
ous and  alternating  currents  are  needed,  whereas  only  one  is 
at  hand.  For  all  electrolytic  operations,  for  most  railway 
work  at  present,  for  telegraphy,  and  sometimes  for  arc  light- 
ing, continuous  current  is  necessary,  while  alternating  current 
is  necessary  for  convenient  application  to  electric  furnaces, 
electric  welding,  electro-cautery  and  other  minor  purposes. 
So  whichever  kind  of  current  is  transmitted  the  other  must  be 
derived  from  it  for  certain  uses. 

All  devices  for  thus  changing  alternating  to  direct  currents, 
or  vice  versa,  with  or  without  accompanying  change  of  voltage, 
may  properly  be  called  current  reorganizers. 

Three  classes  of  such  apparatus  have  come  into  considerable 
use:  1.  Commutators;  2.  Motor  dynamos;  3.  Rotary  con- 
verters. These  classes  are  quite  distinct  from  each  other; 
each  has  advantages  and  faults  peculiar  to  itself,  and  all  three, 
especially  the  last  named,  are  in  e very-day  practical  use  to  a 
greater  extent  than  would  seem  probable  at  first  thought. 

We  have  already  looked  into  the  matter  of  commutation  in 
Chapter  I,  and  have  seen  how  the  naturally  alternating  cur- 
rents in  a  continuous  current  dynamo  are  rectified  and 
smoothed.  Given,  then,  an  alternating  current  received  from 
a  distant  generator,  and  it  would  seem  an  easy  matter  to- 
receive  this  ciurent  upon  a  commutator  and  deliver  it  as  con- 
tinuous current.  In  point  of  fact  there  are  very  serious  diffi- 
culties in  this  apparently  simple  process. 

The  current  received  is  a  set  of  simple  alternations  shown 

280 


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CURRENT  REORGANIZERS,  281 

diagrammatically  in  Fig.  166.  The  figure  shows  three  complete 
periods.  Now,  if  such  a  current  be  sent  into  a  simple  two-part 
commutator,  such  as  is  shown  in  Fig.  9,  Chapter  I,  revolving 
at  such  a  speed  that  the  brushes  will  be  just  passing  from  one 
segment  to  the  other  every  time  the  current  received  changes 
direction,  the  result  will  be  a  rectified  current,  shown  in  Fig. 
167,  unidirectional,  it  is  true,  but  far  from  continuous.    Vari- 


FlG.  166. 

ous  modifications  of  this  simple  rectifying  apparatus  have  been 
and  are  in  extensive  use  for  supplying  current  to  the  field 
magnets  of  alternating  generators.  As  these  machines  are 
generally  multipolar,  the  two-part  commutator  has  been  modi- 
fied so  as  to  reverse  the  current  at  each  alternation.  Fig. 
168  shows  one  of  the  simple  forms  of  commutator  arranged 
for  self-exciting  alternators.  It  consists  of  a  pair  of  metal 
cylinders  mounted  on  and  insulated  from  the  dynamo  shaft. 
Each  cylinder  is  cut  away  into  teeth,  and  the  two  are  moimted 
so  that  the  teeth  interlock  with  insulation  between  them. 
Each  pair  of  consecutive  teeth  acts  like  the  ordinary  two-part 
commutator,  and  there  are  of  course  a  pair  of  teeth  for  every 
pair  of  poles,  so  that  the  commutator  acts  at  each  alternation. 
The  resulting  rectified  current  is  then  led  aromid  the  field 
magnets  of  the  generator,  furnishing  either  the  whole  excita- 
tion, or  enough  to  compoimd  the  machine.    Such  a  current, 


Fio.  187. 

however,  is  so  fluctuating  that  it  is  by  no  means  the  equivalent 
of  an  ordinary  continuous  current  for  magnetizing  purposes, 
hence  in  most  modem  machines  the  main  exciting  current  is 
furnished  by  a  small  exciting  dynamo,  driven  from  the  alter- 
nator shaft  or  by  separate  means,  while  the  rectified  current  is 
used  only  now  and  then  for  compoimding. 

This  simple  current  reorganizer  is  very  successful  for  the 
purpose   described.     But  it   must  be  remembered   that  the 


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282  ELECTRIC  TRANSMISSION  OF  POWER. 

amount  of  energy  concerned  is  trifling,  only  a  very  few  kilo- 
watts being  required  to  compound  even  the  largest  alternators. 
And  despite  this,  there  is  often  trouble  from  sparking,  such 
commutators  being  notoriously  hard  to  keep  in  good  order. 

In  applying  the  same  process  to  rectifying  currents  on  a  larger 
scale,  the  difficulties  from  sparking  are  very  serious,  in  fact 
generally  prohibitive.  And  the  worst  of  it  is  that  they  are 
inherent.  The  root  of  the  trouble  is  that  the  alternating 
current  on  a  line  used  for  general  purposes  cannot  be  kept 
accurately  in  step  with  the  motion  of  the  commutator.  To 
ensure  sparkless  commutation  the  conditions  must  be  as  shown 
in  Fig.  169. 

The  alternations  of  the  current  and  E.  M.  F.  are  shown  by  the 


Fio.  168. 

solid  line,  while  the  brushes  at  the  moment  of  passing  from  one 
commutator  segment  to  the  next  must  take  the  position  b  6, 
with  respect  to  the  current.  That  is,  they  must  pass  from  one 
segment  to  the  next  at  the  moment  when  the  current,  jiLst 
reversing,  is  practically  zero.  So  long  as  the  electromotive 
force  and  the  current  are  in  phase  with  each  other,  as  shown 
in  the  solid  line,  the  current  will  be  rectified  without  trouble- 
some sparking.  But  when  the  current  lags  behind  the 
E.  M.  F.,  as  shown  by  the  dotted  line  of  Fig.  169,  there  is  trouble 
at  once.  The  brushes,  as  can  be  seen  from  the  dotted  pro- 
longations of  b  b,  nmst  break  a  considerable  current,  and  there 
is  certain  to  be  sparking.  Nor  can  any  point  be  found  for  the 
brushes  at  which  they  will  not  have  either  to  break  this  cur- 
rent or  to  pass  from  one  segment  to  the  next  while  there  is 
considerable  E.  M.  F.  between  segments.    The  case  is  bad 


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CURRENT  REORGANIZERS, 


283 


enough  in  a  compounding  commutator  having  a  position  fixed 
with  reference  to  the  E.  M.  F.  of  the  machine  and  dealing 
with  low  voltage  and  moderate  current.  The  inevitable  result 
is  sparking  that  can  be  only  mitigated  by  shifting  the  brushes, 
and  more  or  less  demoralization  of  the  compoimding.  If  the 
current  be  received  from  a  distant  generator  on  a  commuta- 
tor driven  by  a  synchronous  motor,  the  condition  of  things 
is  much  worse.  When  the  current  lags  (or  leads),  not  only 
are  the  brushes  generally  thrown  out  of  step  with  it,  but  if 
there  is  a  sudden  change  of  phase  the  inertia  of  the  commuta- 
ting  apparatus  will  put  it  at  serious  variance  for  the  time  with 
both  current  and  E.  M.  F.  Add  to  this  the  disturbances  of 
phase  produced  by  armature  reaction  in  both  generator  and 
motor,  and  one  has  a  set  of  conditions  that  renders  sparking 


Fio.  169. 


absolutely  certain.  The  most  that  can  be  done  to  help  matters 
is  to  employ  palliative  measures  to  delay  the  destruction  of  the 
commutator.  Aside  from  this  sparking,  it  is  nearly  out  of  the 
question  to  hold  the  voltage  of  the  rectified  current  steady  if 
the  phase  is  shifting,  as  it  often  is  likely  to  be. 

Incidentally  may  be  mentioned  the  fact  that  in  working  such 
a  commutating  apparatus,  just  as  in  rotary  converters,  the 
direction  of  the  rectified  current  will  be  uncertain;  the  brush 
which  happens  to  be  on  a  positive  segment  when  the  brush 
circuit  is  closed,  will  stay  positive,  as  can  readily  be  seen  by 
tracing  out  the  rectifying  process  in  Fig.  168.  In  ordinary 
compoimding  commutators  this  imcertainty  is  absent,  for  with 
the  brushes  in  a  fixed  position  the  positive  segments  will 
always  be  under  the  same  brush,  since  the  segments  are  fixed 
with  reference  to  the  armature  coils. 

No  small  amoimt  of  time  and  money  has  been  spent  in  try- 
ing to  work  out  a  successful  synchrcmizing  commutator.  The 
main  trouble  is,  of  course,  sparking,  and  the  exasperating  part 


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284  ELECTRIC  TRANSMISSION  OF  POWER, 

of  the  problem  is  that  while  on  a  small  scale,  as  in  compound- 
ing alternators,  fair  results  can  be  obtained,  the  difficulties 
increase  enormously  with  the  output,  so  that  every  attempt  on 
a  scale  really  worthy  of  serious  consideration  has  ended  in 
discouragement  and  the  scrap  heap. 

The  great  usefulness  of  such  apparatus  if  of  reasonably  good 
qualities,  has  made  this  field  of  experimentation  very  interest- 
ing, and  a  vast  amount  of  ingenuity  has  been  expended  in 
elaborately  devised  plans  for  reducing  sparking  and  mini- 
mizing the  evil  results  of  shifting  phase.  An  example  of  such 
work,  of  more  than  usual  merit,  was  shown  at  the  International 
Congress  of  1893  at  Chicago.     This  was  the  current  reorganizer 


PlO.  170. 

devised  by  C.  Pollak,  for  use  in  connection  with  accumulator 
installations.  It  was  intended  specifically  for  charging  accum- 
ulators, and  is  very  ingeniously  adapted  to  that  use.  Its 
general  appearance  is  shown  by  Fig.  170.  The  apparatus 
consists  of  a  small  synchronous  motor  driving  a  commutator, 
which  has,  m  the  example  shown,  eight  segments  coupled  alter- 
nately in  parallel  so  as  to  produce  the  effect  of  Fig.  168.  The 
Pollak  commutator  is,  however,  peculiar  in  that  the  spaces 
between  segments  are  of  nearly  the  same  width  as  the  segments 
themselves,  while  the  collecting  brushes  are  set  in  pairs,  so 
that  by  setting  one  of  each  pair  ahead  of,  or  behind  the  other, 
the  ratio  of  segment  width  to  space  width  can  be  changed.     In 


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CURRENT  REORGANIZERS,  286 

charging  accumulators  the  E.  M.  F.  of  the  charging  current 
must  always,  to  prevent  waste  of  energy,  exceed  the  coimter 
E.  M.  F.  of  the  battery.  Hence  a  current  rectified  as  in  Figs. 
167  and  168  cannot  successfully  be  used.  The  arrangement  of 
segments  jiust  described  enables  the  brushes  to  be  so  set  that 
contact  with  a  segment  is  made  at  the  moment  when  the  rising 
E.  M.  F.  of  the  alternating  side  is  exactly  equal  to  the  counter 
E.  M.  F.  of  the  battery,  and  broken  when  the  falling  E.  M.  F. 
reaches  the  same  value.  Only  that  part  of  the  current  wave 
of  which  the  E.  M.  F.  exceeds  the  counter  E.  M.  F.  of  the 
battery  is  used,  the  charging  circuit  being  open  during  the 
remainder  of  the  period.  When  well  adjusted  and  used  on 
a  circuit  nearly  non-inductive,  the  machine  in  question  is 
almost  sparkless  and  very  well  adapted  for  the  particular  pur- 
pose intended.  It  is  also  highly  efficient,  the  only  losses  being 
those  in  the  motor,  plus  brush  friction.  The  total  amount  of 
these  need  be  but  trifling,  probably  less  than  5  per  cent  of  the 
output. 

But  such  apparatus  cannot  be  considered  as  a  general 
solution  of  the  problem,  for  while  quite  successful  for  an 
output  of  10  KW  or  so,  it  has  not  been  tested  in  large  sizes, 
nor  under  the  conditions  of  inductance  ordinarily  to  be  ex- 
pected on  a  power  transmission  circuit.  For  the  reasons 
already  adduced  the  chances  for  success  are  not  good,  particu- 
larly since  all  questions  of  sparking  become  very  grave  when 
large  currents  nmst  be  dealt  with.  This  difficulty  is  well 
known  in  dynamo  working.  For  instance,  in  an  arc  machine 
there  may  be  frequent  recurrence  of  the  long,  wicked-looking 
blue  sparks  familar  to  every  dynamo  tender,  without  notice- 
able damage  to  the  commutator,  while  in  a  low  voltage 
generator  sparking  of  much  less  formidable  appearance  may 
put  the  machine  out  of  business  in  a  very  short  time. 

Bearing  all  this  in  mind,  it  is  but  natural  to  expect  that 
another  particular  solution  of  the  reorganizing  problem  might 
be  foimd  for  arc  lighting.  Here  the  irregularity  of  a  "recti- 
fied" current  is  of  small  consequence,  while  the  small  amount 
of  current  cannot  cause  really  destructive  sparking  if  other  con- 
ditions are  fairly  favorable.  So  it  is  that  we  find  commutating 
apparatus  in  quite  successful  use  for  arc  lighting  in  connection 


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286 


tJLECTRiC  TRANSMISSION  Of  POWER. 


with  alteniating  stations.  The  form  of  apparatus  shown  in 
Fig.  171,  designed  by  Ferranti,  has  been  introduced  in 
several  British  stations  with  good  results.  The  comniutating 
mechanism  is  of  course  used  in  connection  with  a  "constant 
current"  transformer,  arranged  so  as  automatically  to  hold 
the  current  closely  imiform  under  all  variations  of  load. 
Each  conmmtating  unit  supplies  two  separate  arc  circuits  of 
moderate  capacity  —  twelve  lights  in  each.  How  well  the  same 
device  works  at  several  times  the  E.  M.  F.  necessary  to  supply 
so  small  a  series,  is  now  being  demonstrated.  The  present 
tendency  in  central  station  practice  is  to  employ  very  high 


FiQ.  m. 

voltages  for  arc  lighting  —  50  to  100  or  125  lamps  in  series, 
thus  greatly  simplifying  both  the  station  equipment  and  the 
circuits.  The  rectifier  should  at  least  be  able  to  replace  the 
smaller  generators  now  in  use,  and  such  machines  are  now  built 
for  as  many  as  sixty  lights.  This  is  probably  practical  —  in 
fact  there  seems  to  be  no  good  reason  why  the  rectifier  should 
not  be  entirely  available  wherever  it  is  desirable  to  work 
series  arc  circuits  in  connection  with  a  transmission  plant. 
Although  not  in  use  sufficiently  long  to  enable  one  to  pass  a 
final  judgment,  the  machine  is  at  least  promising  and  worth 
careful  investigation.  There  seems  to  be  some  doubt  as  to  the 
successful  working  of  these  rectifiers  at  anything  except  rather 
low.  frequencies,  30  to  40^  or  less,  but  such  a  difficulty  would 


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CURRENT  REORGANIZERS.  287 

appear  to  be  constructional  rather  than  inherent.  It  is  possi- 
ble that  the  alternating  arc  lamp  will  be  developed  far  enough 
to  render  continuous  current  arcs  entirely  unnecessary,  but  this 
remains  yet  to  be  proved,  although  the  inclosed  alternating  arc 
now  gives  highly  successful  results,  particularly  in  street 
lighting. 

All  rectifjdng  commutators  now  in  practical  service  are  of 
very  limited  output  —  not  much  exceedhig  10  to  20  KW,  an 
amoimt  merely  trivial  so  far  as  large  enterprises  are  concerned. 
For  railway  work  or  incandescent  lighting,  these  very  interest- 
ing machines  cannot  be  considered  in  the  race  at  present.  The 
general  problem  is  as  yet  unsolved  by  such  means,  useful  as 
they  may  be  for  special  purposes. 

The  current  delivered  by  rectifiers  is  in  a  measure  discon- 
tinuous, and,  hence,  is  not  the  full  equivalent  of  an  ordinary 
continuous  current.  The  PoUak  machine,  however,  which  is 
intended  to  be  used  with  a  somewhat  flat-topped  alternating 
current  wave,  has  been  successfully  employed  for  working 
motors  as  well  as  for  charging  accumulators.  It  is  not  impos- 
sible that  such  apparatus  may  yet  be  constructed  of  sufficient 
capacity  to  be  of  much  practical  service,  although  the  difficul- 
ties, as  has  already  been  pointed  out,  are  very  considerable, 
and  of  a  kind  very  hard  to  overcome.  Of  course,  polyphase 
currents  can  be  rectified  by  following  the  same  process  as  with 
monophase  current,  and  a  successful  apparatus  would  often 
find  some  place  in  transmission  plants. 

The  advantages  of  the  rectifying  commutator  are  simplicity, 
efficiency,  and  cheapness,  particularly  the  last.  The  working 
parts  are  a  small  synchronous  motor,  made  self-exciting  (and 
self-starting)  by  a  commutator,  and  one  or  more  rectifying 
commutators  driven  by  this  motor.  To  obtain  100  KW  out- 
put, it  is  not  necessary,  as  in  other  forms  of  current  reorgan- 
izers,  to  have  a  machine  nearly  as  large  and  costly  as  a  100 
KW  dynamo.  On  the  contrary,  a  one  or  two  horse-power 
motor  would  be  amply  powerful  to  drive  the  commutator, 
and  the  whole  affair  could  hardly  cost  a  quarter  as  much  as 
a  d)aiamo  of  the  ^  same  capacity,  besides  being  of  greater 
efficiency,  particularly  at  partial  loads.  But  a  hundred  kilo- 
watts is  far  beyond  the  output  of  any  rectifier  that  has  yet  been 


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288  ELECTRIC  TRANSMISSION  OF  POWER, 

put  to  commercial  service,  and  even  a  hundred  kilowatts  is  but  a 
fraction  of  the  output  that  is  often  desirable  in  a  single  imit. 

On  the  other  hand,  a  rectifier  must  require  at  least  the  same 
care  as  a  dynamo,  and  must  in  every  practical  case  be  employed 
in  connection  with  reducing  transformers  to  bring  the  alter- 
nating current  to  the  right  voltage.  The  regulation  too,  is 
somewhat  dubious,  since  compound  winding  is  out  of  the 
question.  And  the  current  is  at  best  disjointed,  likely  to 
produce  needless  hysteresis,  and  of  a  character  rather  hard  to 
measure  conveniently. 

To  sum  up,  the  rectifying  commutator,  while  quite  good 
enough  for  certain  particular  purposes,  has  so  far  given  no 
definite  promise  of  general  usefulness.  All  of  the  serious 
attempts  to  develop  it  on  a  considerable  scale  have  ended  in 
failure.  It  is  not  effectively  reversible,  so  that  the  task  of 
converting  continuous  to  alternating  currents  is  quite  beyond 
it.  While  the  cheapness,  lightness,  and  efficiency  of  such 
apparatus  puts  it  in  these  particulars  far  ahead  of  any  other 
type  of  current  reorganizer,  the  verdict  of  experience  has  so 
far  been  adverse  in  spite  of  these  advantages,  and  engineers 
have  been  driven  to   other   and  more  cumbersome  devices. 

The  most  obvious  method  of  deriving  continuous  from  alter- 
nating currents,  is  to  employ  an  alternating  current  motor  in 
driving  a  continuous  current  dynamo.  The  two  machines 
may  be  connected  in  any  convenient  way,  by  belting,  clutching 
the  shafts  together,  or  by  putting  them  in  even  more  intimate 
connection  by  placing  two  armatures  on  the  same  shaft  or  two 
windings  on  the  same  core. 

The  procedure  first  mentioned  is  not  infrequent,  particularly 
when  a  transmission  of  power  plant  is  installed  in  connection 
with  an  existing  lighting  or  power  station.  A  synchronous 
motor  is  installed  in  place  of  the  previously  used  engines, 
belted  m  any  convenient  way  to  the  existing  generators, 
and  the  operation  of  the  station  goes  on  as  before.  Further 
description  is  unnecessary,  as  the  apparatus  is  in  no  way  out 
of  the  ordinary,  and  not  at  all  specialized  for  the  conversion  of 
alternating  to  continuous  currents.  As  a  rule  such  installa- 
tions have  temporary,  and  have  been  replaced  later  by  special 
apparatus  worked  directly  from  the  transmission  system. 


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CURRENT  REORGANIZERS.  289 

A  more  interesting  way  of  accomplishing  the  same  result  is 
by  the  use  of  a  twin  machine  comprising  motor  and  generator 
on  the  same  bed  plate,  or  even  on  the  same  shaft.  In  this  way 
the  reorganizing  apparatus  is  formed  into  a  compact  unit, 
convenient  to  install  and  to  operate,  and  possessing  an  effi- 
*ciency  higher  than  that  of  two  belted  machines,  by  the  belt 
losses  and  more  or  less  of .  the  bearing  friction.  The  total 
increase  of  efficiency  is  perhaps  5  per  cent,  when  the  com- 
parison is  between  a  pair  of  coupled  machines  and  a  pair 
directly  belted,  or  more  if  the  belting  be  indirect.     Moreover, 


PlO.  172. 

the  motor  and  dynamo  parts  of  the  machine  can  each  be 
designed  so  as  to  give  the  best  efl&ciency  and  economy  of 
construction  possible  at  the  given  mutual  speed.  A  unit  of 
this  class  is  shown  in  Fig.  172  —  an  early  Siemens  continuous 
alternating  transformer.  The  motor  part  is  wound  for  2,000 
volts,  monophase,  and  the  dynamo  part,  of  the  well-known  Sie- 
mens internal  pole-type,  with  overhung  armatiu-e  and  brushes 
directly  on  the  windings,  delivers  continuous  current  at  150 
volts.  In  this  case  the  machine  has  three  bearings,  although 
in  many  cases  it  would  be  quite  possible  to  get  along  with 
two.  The  main  advantage  of  this  duplex  form  of  machine  is 
the  complete  independence  of  the  two  component  parts  in 


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290  ELECTRIC  TRANSMISSION  OP  POWER. 

their  electrical  relations.  The  motor  part  can  be  designed 
for  any  desired  voltage  or  number  of  alternations.  It  can 
often,  except  in  very  long  transmissions,  take  the  line  voltage 
directly  without  need  for  reducing  transformers,  while  the 
number  of  alternations  can  be  chosen  solely  with  reference  to 
general  conditions  and  without  considering  the  direct  cmrent ' 
end  of  the  machine  at  all.  This,  as  will  be  seen  when  we  have 
considered  some  other  types  of  current  reorganizers,  is  a  very 
valuable  property,  since  it  gives  the  power  of  obtaining  con- 
tinuous current  in  a  thoroughly  practical  way  from  alternating 
currents  of  any  frequency.  Other  reorganizers  can  be  worked 
to  advantage  only  within  a  somewhat  limited  range  of  fre- 
quency. Again,  the  motor  dynamo  can  be  compounded  on 
the  continuous  current  side  without  in  any  way  reacting  upon 
the  alternating  circuit,  and  the  two  circuits  can  be  regulated 
independently  in  any  desired  manner.  All  difficulties  due  to 
lagging  current  can  be  eliminated,  and  the  continuous  ciurent 
side  can  be  kept  at  constant  pressure  irrespective  of  loss  in 
the  main  line  or  any  variations  of  voltage  or  phase  occur- 
ring in  it. 

Finally,  the  apparatus  can  as  readily  give  alternating  current 
from  continuous,  as  the  reverse,  and  with  the  same  indepen- 
dence in  each  case. 

The  compensating  disadvantages  are  high  first  cost  and 
rather  large  loss  of  energy  in  the  double  transformation.  As 
to  the  former  count,  it  may  be  said  that  the  advantages  gained 
in  possible  range  of  frequency  and  flexibility  in  the  matter  of 
voltage  go  far  to  offset  the  increase  of  cost.  Often  such 
a  motor  dynamo  is  the  only  possible  way  of  securing  the 
necessary  current.  For  example,  if  one  wished  continuous 
current  for  heavy  motor  service,  such  as  hoists  and  the  like, 
where  the  only  current  available  was  monophase  alternating 
of  125^,  or  even  of  60-^  for  that  matter,  the  motor  dynamo 
would  be  the  only  practical  way  of  solving  the  problem. 

As  regards  efficiency  the  motor  dynamo  should  be,  and  is, 
a  little  better  than  motor  and  dynamo  separately,  owing  to 
lessened  friction  of  the  bearings.  Its  efficiency  should  be  as 
great  as  85  per  cent  at  full  load,  and  might  easily  be  2  or 
3  per  cent  higher,  in  large  machines.     At  half  load  it  should 


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Fig.  1, 


FlQ.  2. 


PLATE  IX. 


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CURRENT  REORGANIZERS,  291 

be  say  82  to  85  per  cent.  Practice  too  often  shows  results 
several  per  cent  below  those  mentioned,  but  this  is  because 
motor  dynamos  have  usually  been  of  very  small  size  and 
sometimes  have  been  made  up  from  any  machines  of  the  right 
speed  that  were  at  hand. 

The  usual  synchronous  motor  may  In  small  motor  genera- 
tors be  replaced  to  advantage  by  an  induction  motor,  which 
is  simpler  than  the  s3rnchronous  form  and  requires  no  brushes. 
Such  a  combination  is  shown  in  Fig.  173.     This  machine  is 


Fig.  173. 

specially  designed  for  furnishing  charging  current  for  auto- 
mobile batteries.  Through  most  residence  districts  only  alter- 
nating current  is  available,  and  the  convenience  of  such  an 
apparatus  is  very  great. 

The  motor  is  a  monophase  induction  machine  of  the  class 
shown  in  Plate  VIII,  Fig.  2,  suited  to  ordinary  lighting  circuits. 
Of  late  such  machines  have  assumed  considerable  importance, 
and  many  large  imits  have  been  produced.  Plate  IX  shows  in 
Fig.  1  a  500  KW  quarter-phase  set  running  at  400  r.  p.  m.  It 
consists  of  an  8  pole  500  KW  railway  generator  coupled  directly 
to  a  20  pole  2,200  volt  synchronous  motor,  the  two  machines 


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Fig.  1. 


FiQ.  2. 


PLATE  IX. 


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CURRENT  REORGANIZERS. 


291 


be  say  82  to  85  per  cent.  Practice  too  often  shows  results 
several  per  cent  below  those  mentioned,  but  this  is  because 
motor  dynamos  have  usually  been  of  very  small  size  and 
sometimes  have  been  made  up  from  any  machines  of  the  right 
speed  that  were  at  hand. 

The  usual  synchronous  motor  may  hi  small  motor  genera- 
tors be  replaced  to  advantage  by  an  induction  motor,  which 
is  simpler  than  the  synchronous  form  and  requires  no  brushes. 
Such  a  combination  is  shown  in  Fig.  173.     This  machine  is 


X 


I  f  /i 


FlO.  178. 

specially  designed  for  furnishing  charging  current  for  auto- 
mobile batteries.  Through  most  residence  districts  only  alter- 
nating current  is  available,  and  the  convenience  of  such  an 
apparatus  is  very  great. 

The  motor  is  a  monophase  induction  machine  of  the  class 
shown  in  Plate  VIII,  Fig.  2,  suited  to  ordinary  lighting  circuits. 
Of  late  such  machines  have  assumed  considerable  importance, 
and  many  large  units  have  been  produced.  Plate  IX  shows  in 
Fig.  1  a  500  KW  quarter-phase  set  running  at  400  r.  p.  m.  It 
consists  of  an  8  pole  500  KW  railway  generator  coupled  directly 
to  a  20  p)ole  2,200  volt  synchronous  motor,  the  two  machines 


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292  ELECTRIC  TRANSMISSION  OP  POWER, 

having  a  common  bearing  between  them.  An  interesting 
feature  of  this  set  is  the  exciter  mounted  on  the  same  shaft,  an 
8  KW  multipolar  generator,  so  that  the  whole  outfit  is  self- 
contained.  The  frequency  in  this  case  is  66-^,  a  periodicity  at 
which  such  motor  generators  have  a  material  advantage  over 
other  apparatus  for  a  like  purpose. 

Fig.  2  is  out  of  the  ordinary  in  that  the  motor  is  of  the 
induction,  type,  instead  of  the  ordinary  synchronous  machine. 
The  set  shown  is  of  100  K W  output,  and  comprises  an  ordinary 
6  pole  600  volt  railway  generator  coupled  to  a  12  pole  three- 
phase  induction  motor,  running  at  600  r.  p.  m.,  the  periodicity 
being  60^.  Induction  motors  have  recently  come  into  consid- 
erable use  in  this  sort  of  work,  in  spite  of  somewhat  lower  effi- 
ciency than  the  corresponding  synchronous  motors.  It  is  safe  to 
say  that  the  difference  in  efficiency  is  2  or  3  per  cent,  and  while 
the  S3mchronous  motor  may  be  overexcited  so  as  to  improve 
the  power  factor  of  the  system,  the  induction  motor  always 
introduces  lagging  current.  Yet  a  number  of  motor  generators 
with  induction  motors  are  now  being  built  of  capacity  from  500 
to  nearly  1,000  KW.  The  real  reason  for  the  use  of  induction 
motors  on  so  large  a  scale  is  the  trouble  which  has  been  experi- 
enced at  many  times  and  places  from  hmiting.  These  troubles 
do  not  get  widely  advertised  outside  the  stations  where  they 
occur,  but  it  is  a  fact  that  in  the  use  of  rotary  converters  and 
synchronous  motors  on  a  large  scale  very  serious  and  formi- 
dable developments  of  this  phenomenon  have  occurred,  so  that 
in  spite  of  the  use  of  shields  it  has  under  certain  conditions, 
especially  when  incandescent  lighting  circuits  were  to  be  fed, . 
seemed  wise  to  have  recourse  to  induction  motors.  It  is,  how- 
ever, probably  best  to  regard  this  as  a  temporary  expedient, 
as  synchronous  motors,  at  least,  can  be  practically  freed  from 
hunting  by  proper  design  and  construction,  and  possess  very 
considerable  advantages.  The  demand  for  machines  of  extreme 
multipolar  construction,  a  demand  based  largely  on  fashion, 
and  the  use  of  laminated  pole  pieces,  are  responsible  for  a 
good  share  of  the  trouble.  Rotary  converters,  as  we  shall  pres- 
ently see,  present  even  more  serious  problems. 

In  these  large  motor  dynamos  it  is  possible  to  reach  full  load 
efficiencies  in  the  neighborhood  of  90  per  cent,  and  figures 


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CURRENT  REORGANIZERS. 


293 


fully  up  to  that  point  have  actually  been  obtained.  As  large 
synchronous  motors  can  readily  be  wound  for  10,000  or  12,000 
volts,  under  favorable  conditions  motor  dynamos  can  be  used 
without  reducing  transformers,  which  averts  a  loss  of  2.5  or  3 
per  cent,  that  would  otherwise  be  incurred. 

From  the  duplex  machines  just  described  it  is  but  a  short 
step  to  the  composite  dynamotor,  so  called,  of  which  the 
armature  is  double  wound.  The  primary  or  high  voltage 
winding  may  of  course  be  either  altematmg  or  continuous. 


Fig.  174. 

The  secondary  winding  is  likewise  for  either  current,  and  may 
well  be  fitted  with  both  commutator  and  collecting  rings. 
A  favorite  arrangement  of  the  windings  is  to  place  the 
secondary  coils  in  slots  in  the  armature  core,  apply  a  sheath- 
ing of  insulation,  and  then  to  wind  the  primary  coils  on  the 
smooth  surface  thus  formed.  The  commutators  or  rings 
are  placed  one  at  each  end  of  the  armature,  as  in  the  con- 
tinuous current  transformer  shown  in  Fig.  37,  Chapter  III. 

A  typical  dynamotor  of  this  sort  is  shown  in  Fig.  174.  This 
is  specifically  intended  to  derive  a  high  voltage  alternating  cur- 
rent for  testing  purposes  from  a  low  voltage  continuous  cur- 
rent.    The  output  is  small,  only  a  fraction  of  a  kilowatt 


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294 


ELECTRIC  TRANSMISSION  OF  POWER, 


and  the  armature  is  in  the  ordinary  bipolar  field  used  for  small 
motors.  The  motor  or  primary  winding  is  for  110  volts, 
continuous,  and  the  secondary  for  5,000  volts,  alternating.  Of 
course  these  voltages  might  be  anything  desirable,  since  in  so 
small  a  machine  there  are  no  difficulties  in  the  way. 

Another  excellent  specimen  of  the  same  type  is  Fig.  175,  a 
Lahmeyer  **umformer'*  of  about  30  KW  output.  It  is  pri- 
marily a  continuous  current  transformer,  with  675  volts  primary 
and  115  volts  secondary.     It  is  fitted,  however,  as  shown  in 


FlO.  175. 

the  cut,  with  collector  rings  outside  one  of  the  bearings,  from 
which  three-phase  current  at  about  70  volts  can  be  taken. 
There  are  four  field  poles,  and  as  the  normal  speed  is  850 
revolutions  per  minute,  the  three-phase  current  is  at  a  fre- 
quency of  a  little  less  than  30-w  per  second. 

This  was  one  of  the  machines  exhibited  at  the  Frankfort 
Exposition  of  1891,  and  fortunately  an  efficiency  test  of  it  is 
available,  dealing,  however,  only  with  continuous  currents. 
From  the  nature  of  the  case  the  efficiency  with  a  three-phase 
secondary  would  not  differ  substantially  from  that  found,  so 


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CURRENT  REORGANIZERB. 


295 


that  the  curve,  Fig.  176,  gives  a  closely  approximate  idea  of  the 
general  efficiency  of  such  apparatus  in  the  smaller  sizes.  At 
full  load  the  commercial  efficiency  is  very  nearly  85  per  cent, 
while  at  half  load  it  has  dwindled  to  77  per  cent.  This  is  not 
bad  for  a  small  machine,  and  in  a  unit  of  100  KW  or  more  could 
undoubtedly  be  raised  several  per  cent.  It  should  be  at  least  as 
high  as  can  be  obtained  from  a  duplex  motor  d3aiamo,  in  fact 
rather  higher,  since  the  bearing  friction  and  core  losses  are 
diminished.  The  composite  machine  is  also  cheaper,  since  but 
one  field  is  \ised,  and  it  has  a  certdn  advantage  in  that  the  arma- 
ture reactance  due  to  the  motor  and  dynamo  windings  tend  to 


81 


10 


80K.MI 


Fio.  176. 


oppose  each  other,  and  hence  to  diminish  possible  sparking  and 
disturbance  of  the  field.  It  has  the  same  independence  of  pri- 
mary and  secondary  voltage  as  the  duplex  motor  dynamo. 
On  the  other  hand,  by  reason  of  a  common  field,  the  period- 
icity of  the  currents  in  both  windings  must  be  the  same.  It 
must  be  remembered  that  a  continuous  current  armature  has  a 
periodicity  just  as  truly  as  an  alternating  armature.  The  cur- 
rent as  generated  in  each  is  alternating,  but  in  the  former  it  is 
commuted  before  leaving  the  generator.  Now,  the  frequency 
of  these  alternations  depends  directly  on  the  nmnber  of  poles 
and  the  revolutions  per  minute,  being  in  fact  the  numerical 
product  of  the  two.  So  if  one  of  these  composite  d3aiamotors 
be  \ised  with  the  continuous  current  winding  as  primary,  the 


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296  ELECTRIC  TRANSMISSION  OF  POWER. 

frequency  of  the  alternating  secondary  is  fixed,  since  the 
speed  of  the  machine  cannot  be  changed  without  involving 
both  primary  and  secondary  voltages.  If  the  alternating  cur- 
rent side  be  used  as  the  primary,  the  speed  of  the  machine  is 
•fixed  by  the  number  of  alternations,  and  whatever  the  voltage 
of  the  secondary,  the  frequency  must  be  the  same  as  that  of 
the  primary.  Now  it  is  a  fact  well  known  to  dynamo  designers, 
that  continuous  current  dynamos  generating  a  high  frequency 
current  prior  to  its  commutation  are  troublesome  and  costly 
to  build.  Most  continuous  current  dynamos  have  an  intrinsic 
frequency  of  15  to  25-^  per  second.  *  To  increase  these  figures 
to  40^  involves  some  difficulty,  particularly  in  large  machines, 
while  50  to  60^  are  rather  hard  to  reach,  unless  in  sizes  of 
100  KW  and  below. 

Hence,  in  spite  of  the  good  points  of  the  composite  dyna- 
motor,  it  is  of  limited  utility  compared  with  the  dujJex  machine 
previously  described,  particularly  since  there  is  a  simpler  way 
of  doing  the  same  work  with  a  higher  efficiency. 

This  is  found  in  the  so-called  rotary  or  synchronous  converter, 
now  used  on  a  very  large  scale. 

This  machine  is  nothing  more  than  a  continuous  current 
dynamo  fitted  with  collecting  rings  in  addition  to  ijie  com- 
mutator. These  rings  are  connected  to  appropriate  points  of 
the  armature  winding,  and  supplied  with  alternating  currents 
of  the  same  frequency  which  would  be  generated  by  the  arma- 
ture if  the  machine  were  used  as  a  dynamo.  The  brushes 
being  raised,  the  machine  is  nothing  but  a  synchronous  motor 
running  without  load  at  its  normal  speed.  Now,  when,  the 
brushes  are  put  down,  the  alternating  current  simply  flows 
through  the  armature  much  as  if  it  were  generated  therein,  is 
commuted  and  passes  out  upon  the  line.  This  commutation 
takes  place  under  just  the  same  general  conditions  as  if  the 
machine  were  used  as  a  generator.  Meanwhile  a  portion  of 
the  current  supplied  is  passing  as  before,  not  through  the 
brushes  but  through  the  'wdnding  to  the  collecting  rings,  keep- 
ing up  the  action  as  a  motor.  Of  the  total  current  then,  a 
small  part  forces  its  way  against  the  E.  M.  F.  set  up  in  the 
windings  by  the  field,  and  supplies  the  motor  function;  a  far 
greater  part,  in  amount  determined  by  the  resistance  and 


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CURRENT  REOROANIZERS.  297 

inductance  of  the  armature,  flows  as  if  urged  by  this  E.  M.  F., 
to  the  brushes,  and  supplies  the  generator  function  of  the 
machine.  But  a  single  armature  winding  serves  to  drive  the 
armature  and  to  furnish  a  large  output  of  commutated  current. 
And  this  current  is  not  simply  rectified,  but  is  of  exactly  the 
same  character  as  if  generated  in  the  armature. 

Inasmuch  as  the  armature  is  revolving  in  a  magnetic  field, 
the  transfer  of  energy  through  the  rotary  converter  is  not  in 
the  last  analysis  a  case  of  pure  conduction  and  commutation. 
A  part  of  the  energy  spent  in  the  motor  part  of  the  armature 
goes  into  dynamical  increase  of  output  in  the  part  which 
for  the  moment  acts  as  generator.     There  is  thus  a  motor 


FlO.  177. 

generator  action  in  the  same  armature.  Of  the  total  energy 
delivered  from  the  d.c.  side  in  a  monophase  converter  like 
Fig.  177,  a  little  more  than  40  per  cent  of  the  energy  is  dynam- 
ically transferred,  in  the  polyphase  forms  much  less,  say 
12  to  24  per  cent  according  to  the  number  of  armature  taps. 
The  required  motor  activity  in  the  converter  is  thus  consider- 
ably in  excess  of  that  required  merely  to  spin  the  armature  at 
synchronous  speed. 

The  character  of  the  winding  in  a  rotary  converter  is  gen- 
erally precisely  the  same  as  in  a  continuous  current  generator, 
the  only  addition  being  two  or  more  leads  from  symmetrically 
placed  iK)ints  in  the  winding  to  the  collecting  rings.     These  leads 


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298 


ELECTRIC  TRAXSML<SIOX  OP  POWER. 


can  l)c  8o  arrangoil  as  lo  :\tti:  a  : 
natinj?  current  or,  if  desire i.  a  iii 
lattcT  forms  are  trenersilly  pTviV 
in^  HVnchrtMious  ino^»r>  :hty  c 
1  ho  in(>noph».<o  niaohir.e  h:t>  : 
and  by  no  nu\Hns  s^i:v.]»'o  r..t  :hxi: 
of  the  armature  in  a  s::..yrie  1 
phiiHo).  Here  the  ov^nur.u.us 
rlnj?  in  16  ^vtions,  F^'n:  ;be 
mwi  may  Ix*  applicii  or  ^iibi 


::•  Tj-pbase  sr-stem  for  the  alter- 
.»- « 'T  ihr^ee-phase  system.  The 
rr^i,  fdi^ce  like  the  correspond- 
sii.  t:»e  n^ie  self -starting,  while 
l»e  r.r  urhi  to  speed  by  special 
N.  Tji.  177  sbo-ws  the  character 
•r.v  ili^  r  TATV  converto"  (mono- 
rur!^i.i  '•ii.iinfi:  is  a  Gramme 
rrusbfs  B.  B,  continiiotis  ctir- 
r.  i«  bile  ibe  brushes  on  the  col- 


liM-hh^i  vin»x  (\  (\  <|v->-,  -  -Sf  ^,^^  ,f  .^.•f  f. ,y  ^^  altematinj; 
rnuiMU  S\io]\  ^'^  )  :i,\,  c  "  -\  st^'-vT  h  \'h.Tii\y  of  puTposes  as 
h»ll.u\N  \  ron;\^;j,Niv  o..-^.- :  /.y  j.  ».  .  2,  Ahemating  ctiT- 
ivMt  d\ujnu,N.  :^  v\v  ,,  ;v.;^i  .-.—:-::■':  'T.  4,  Synchronous 
♦dhMuoHn^i    t\)oi,N^       ,\    0.':  •  '   ^    i>  :/':*7:..s:lr.i:  e:»r.verter.     6. 

i^«p'»-^'^  tN>iM'\  .N.'v.-..-^  j.-Y  ..^  :.'-  y  >,  ppbei  with  four 
'"^'"'<''^'''  '»'  ^^  ^^^-  -.N  a:  :,^  :..--  7-,-  .".-•..,:<,  oAch  oi^e  i<nn- 
*""  ''*'^  ^^•M.i,^.v  ...  ,^,,.  ,  ,,  ,^  .,  -  ,.  .~:.^ -<  .,f  ^y^^  armatuie. 
'"1^'"^'^   tMnv,^,    .V   ^.    ,,..    ,    v.v,    ;:.->,^.    c  Gt^riiT^s  rings. 


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298 


ELECTRIC  TRANSMISSION  OF  POWER. 


can  be  so  arranged  as  to  form  a  monophase  system  for  the  alter- 
nating current  or,  if  desired,  a  two-  or  three-phase  system.  The 
latter  forms  are  generally  preferred,  since  like  the  correspond- 
ing synchronous  motors  they  can  be  made  self-starting,  while 
the  monophase  machine  has  to  be  brought  to  speed  by  special 
and  by  no  means  simple  methods.  Fig.  177  shows  the  character 
of  the  armature  in  a  simple  bipolar  rotary  converter  (mono- 
phase). Here  the  continuous  current^  wdnding  is  a  Gramme 
ring  in  16  sections.  From  the  brushes  B,  B,  continuous  cur- 
rent may  be  applied  or  withdrawn,  while  the  brushes  on  the  col- 


Fio.  178. 

lecting  rings  C,  C,  perform  the  same  office  for  the  alternating 
current.  Such  a  machine  may  serve  a  variety  of  purposes  as 
follows:  1.  Continuous  current  dynamo.  2.  Alternating  cur- 
rent dynamo.  3.  Continuous  current  motor.  4.  Synchronous 
alternating  motor.  5.  Continuous  alternating  converter.  6. 
Alternating  continuous  converter. 

Diphase  rotary  converters  are  usually  supplied  with  four 
collecting  rings  connected  to  form  two  circuits,  each  one  join- 
u\£r  the  windings  in  two  opposite  quadrants  of  the  armature. 
Triphase  transformers  generally  have  three  collecting  rings, 


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Fio.  1. 


Fig.  2. 


PLATE  X. 

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CURRENT  REORGANIZERS,  299 

with  their  respective  leads  tapped  into  the  windings  120° 
apart.  The  connections  vary  somewhat  for  different  kinds  of 
armature  windings,  but  are  the  same  in  effect  as  those  just 
indicated.  One  of  the  early  practical  machines  of  this  sort 
exhibited  at  the  Frankfort  Exposition  of  1891  is  shown  in  Fig. 
178.  It  is  of  the  flat  ring  type  usual  to  dynamos  of  Schuckert 
make,  and  is  fitted  with  four  collecting  rings  outside  the  bear- 
ing at  the  commutator  end.  The  rings  were  arranged  for 
either  monophase  or  diphase  connection.  The  rotary  converter 
thus  organized  attracted  great  attention,  and  was  successfully 
operated  in  its  manifold  and  diverse  functions.  It  should 
be  noted  that  if  driven  as  a  dynamo,  such  a  machine  can  furnish 
continuous  and  alternating  current  simultaneously,  a  property 
sometimes  convenient,  and  now  not  infrequently  utilized. 

These  rotary  converters  in  the  diphase  and  triphase  forms 
are  playing  a  very  important  part  in  electric  railway  operations 
involving  considerable  distances,  and  a  large  number  of  them 
are  in  highly  successful  use.  A  good  idea  of  the  modem  tyi>e 
of  rotaiy  converter  is  shown  in  Fig.  2,  Plate  X.  This  is  one  of 
the  400  KW  machines  installed  in  1894  to  operate  the  electric 
railways  in  the  city  of  Portland,  Ore.  It  is  designed  to  deliver 
continuous  current  at  nearly  600  volts,  and  receives  its  energy 
from  Oregon  City,  about  fourteen  miles  away,  where  is  in- 
stalled a  triphase  transmission  plant.  The  motive  power  is 
derived  from  the  great  falls  of  the  Willamette  River.  Current 
is  generated  at  6,000  volts,  with  a  frequency  of  33-^  per  second, 
and  is  given  to  the  rotary  converters  at  about  400  volts, 
from  the  secondaries  of  the  reducing  transformers.  Fig.  1, 
Plate  X,  shows  a  250  KW  Westhighouse  diphase  machine, 
adapted  for  use  on  a  60-^  circuit  and  giving  continuous  current 
at  250  volts.  An  interesting  feature  of  this  machine  is  the 
diphase  induction  motor  with  its  armature  on  an  extension  of 
the  main  shaft.  This  serves  to  bring  the  machine  to  speed 
without  calluig  for  the  excessive  current  that  would  be  required 
if  the  main  lines  were  closed  upon  the  converter  armature 
itself.  The  monophase  form  of  this  very  interesting  apparatus 
has  not  yet  come  into  much  practical  use,  not  through  any  in- 
herent faults,  but  because  most  of  the  power  transmission  has 
so  far  been  accomplished  with  diphase  and  triphase  currents. 


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800  ELECTRIC  TRANSMISSION  OF  POWER. 

The  efficiency  of  these  machines  is,  as  might  be  expected 
from  their  character,  practically  the  same  as  ordinary  con- 
tinuous current  dynamos  of  the  same  output,  or  rather  better 
on  account  of  the  shorter  average  path  for  the  current  in  the 
armature.  In  fact,  so  far  as  general  properties  go,  they  are 
dynamos.  They  furnish  at  present  by  far  the  most  available 
means  of  deriving  continuous  from  alternating  currents,  for 
they  are  simple,  of  great  efficiency,  and  of  about  the  same  price 
as  other  generators  of  the  same  capacity.  In  point  of  fact,  a 
well-designed  polyphase  rotary  converter  has  rather  better 
output  and  efficiency  than  the  corresponding  generator,  since 
for  the  reason  just  noted  the  armature  losses  are  diminished. 
Bearing  this  in  mind,  it  is  apparent  that  increasing  the  number 
of  points  at  which  the  armature  is  tapped  for  the  alternating 
current  supply,  thus  shortening  the  average  path  to  the  brushes, 
will,  other  things  being  equal,  lessen  the  armature  loss.  In 
practice  it  is  found  that  a  three-phase  converter  with 
three  armature  taps  is  considerably  better  than  a  monophase 
converter  with  two;  a  quarter-phase  converter  with  four  is 
somewhat  better  still,  while  a  three-phase  connection  with 
separate  phases  and  six  taps  gives  even  a  higher  output  and 
efficiency.  The  net  result  is  that  while  a  monophase  con- 
verter is  rather  inferior  to  the  corresponding  dynamo  the 
two-  and  three-phase  converters  are  considerably  better  than 
the  corresponding  dynamos.  Quarter-phase  converters  are 
always  connected  for  four  collecting  rings,  and  large  three- 
phase  converters  not  infrequently  have  six,  to  gain  the  advan- 
tage just  mentioned. 

Efficiencies  as  great  as  96  per  cent  at  full  load  have  been 
obtained  from  large  rotary  converters,  with  93.6  per  cent  at 
half  load.  These  figures  are  from  a  three-phase,  six  collect- 
ing ring  converter  of  nearly  1,000  KW  output. 

As  already  indicated,  there  is  a  strong  tendency  toward  the 
use  of  low  periodicity,  25^^  to  30>^  in  rotary  converters.  This 
is  partially  due  to  the  complication  of  the  commutator  in  high 
frequency  converters,  partly  to  the  current  fashion  for 
extremely  low  rotative  speeds,  and  partly  to  lack  of  finesse  on 
the  part  of  the  average  designer.  That  converters  for  a  fre- 
quency as  high  as  60^  are  entirely  feasible  even  in  capacities 


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CURkENT  REORGANIZBRS.  801 

up  to  several  hundred  kilowatts  admits  of  no  discussion,  as 
the  machine  put  in  evidence  in  Plate  X,  of  which  a  number 
are  in  successful  operation,  plainly  shows.  It  is  imdoubtedly 
easier  and  cheaper  to  build  them  for  somewhat  lower  periodici- 
ties, but  there  seems  very  little  reason  for  going  so  low  as  is  the 
current  custom,  and  it  tends  needlessly  to  multiply  special 
types  of  apparatus. 

And  yet  the  simplicity  of  the  rotary  converter  is  attained 
at  the  cost  of  certain  practical  inconveniences  that  cannot 
lightly  be  passed  by.  Their  source  is  the  employment  of  a 
single  field  and  armature  winding  for  all  the  purposes  of  the 
apparatus.  The  results  are,  first,  complete  interdependence 
of  the  alternating  and  continuous  voltages,  and,  second,  con- 
sequent difficulties  of  regulation  that  are  occasionally  very 
troublesome. 

The  immediate  result  of  a  single  winding  is  that  there  is  an 
approximately  fixed  ratio  between  the  alternating  and  the  con- 
tinuous voltage.  The  former  is  always  the  lass,  and,  while 
varied  by  changes  in  the  number  of  phases  determined  by  the 
connections,  is  approximately  the  alternating  voltage  that 
would  be  yielded  by  the  machine  driven  as  a  generator.  This 
is,  for  monophase  or  diphase  connections,  about  seven-tenths 
of  the  continuous  current  voltage,  and  for  three-phase  connec- 
tions about  six-tenths.     The  proportions  would  approximate  to 

1              V3 
/-  and ^  respectively,  if  the  alternating  E.  M.  F.'s  were 

V2  2  V2 

sine  waves,  which  they  never  are  when  derived  from  an  ordi- 
nary continuous  current  armature.  In  service  the  real  pro- 
portions may,  and  generally  do,  vary  by  several  per  cent, 
according  to  the  excitation.  In  a  particular  two-phase  case 
the  actual  ratio  was  .68,  and  in  a  three-phase  case  .65.  If, 
therefore,  a  rotary  converter  be  used  for  supplying  continuous 
current,  the  applied  alternating  current  must  be  of  lower  pres- 
sure than  the  derived  continuous,  in  about  the  proportion 
above  noted.  This  compels  the  use  of  reducing  transformers 
in  every  case  of  power  transmission  involving  this  apparatus. 
Further,  any  cause  that  affects  the  alternating  pressure  affects 
the  continuous  as  well.  Line  loss,  inductance,  resonance  effects, 
as  well  as  changes  at  the  generators,  all  influence  the  vol- 


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302  ELECTRIC  TRANSMISSION  OF  POWER. 

tage  at  the  continuous  current  end  of  the  rotary  transformer. 
Nor  can  this  voltage  be  freely  altered  by  changing  the  field 
strength,  since,  as  we  have  already  seen,  this  may  profoundly 
change  the  inductance  of  the  alternating  circuit,  which  is  for 
many  reasons  undesirable.  The  field  windings  of  rotaries  are 
either  shunt  from  the  d.o.  side  or  compound.  The  former 
winding  gives  much  the  steadier  power  factor  and,  hence, 
is  rather  desirable  for  close  regulation  of  a  steady  load,  while 
the  latter  is  advantageously  used  for  railway  loads  and  the  like. 
The  best  results  are  obtained  by  carefully  adjusting  the  gen- 
erator, line,  and  rotary  converters  to  work  together.  Other- 
vnse  there  is  likely  to  be  trouble  in  regulation. 

For  these  reasons  in  cases  where  close  regulation  is  neces- 
sary, as  for  incandescent  lighting,  preference  has  frequently 
been  given  to  the  motor  generator  with  double  field  and  arma- 
ture, as  in  the  large  Budapest  system  installed  by  Schuckert  & 
Co.,  who  were  among  the  pioneers  in  developing  the  rotary 
converter.  In  this  case  the  transmission  is  at  2,000  volts 
diphase,  at  which  pressure  current  is  delivered  to  the  motor 
end  of  the  motor  generators  placed  in  substations  at  conve- 
nient points.  In  such  a  plant  the  increased  cost  of  the  duplex 
machines  is  not  so  great  as  might  be  supposed,  for  reducing 
transformers  are  needless,  and  the  output  of  both  generators 
and  motors  can  be  forced  to  the  utmost  limit  of  efficient  oper- 
ation, without  fear  of  injuring  the  regulation,  which  is  reduced 
to  the  easy  problem  of  accurately  compounding  a  continuous 
current  generator.  The  net  efficiency  of  the  Budapest  trans- 
formation is  said  to  be  85  per  cent.  Some  recent  experiments 
on  the  relative  efficiency  and  cost  of  motor  generators  and 
rotary  converters  are  as  follows  :  The  sets  compared  were  of 
200  KW  capacity  for  changing  triphase  current  from  the 
Niagara  circuits  at  11,000  volts,  25^  into  continuous  current 
at  120  to  150  volts.  The  efficiencies  given  are  net,  including 
the  necessary  provisions  for  obtaining  a  variation  of  25  per 
cent  in  the  finally  resulting  voltage: 


Motor- 
Oenerator. 

Transformers 
and  Rotaries. 

Difference. 

Full  load 

87.40 

89.87 

2.47% 

f  load 

85.54 

88.70 

3.16% 

}load 

81.42 

84.00 

8.48% 

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CURRENT  REORGANIZERS.  808 

The  extra  apparatus  required  with  the  rotaries  brought  the 
two  methods  to  substantially  the  same  cost,  but  for  lighting 
work  the  motor  generators  gave  the  better  results. 

From  the  foregoing  it  is  sufficiently  evident  that  every  case  of 
current  reorganization  cannot  be  successfully  met  by  the  same 
apparatus.  For  certain  small  work  the  rotating  commutator 
seems  to  be  fairly  weU  suited,  and  for  occasional  purposes  it 
is  somewhat  cheaper  and  more  efficient  than  any  of  its  rivals. 
Next  in  point  of  efficiency  and  cheapness  comes  the  rotary 
converter,  infinitely  better  for  heavy  work  than  any  commutat- 
ing  device,  and  finding  very  extensive  application  to  electric 
railway  work.  Finally,  for  work  requiring  very  close  regulation, 
the  motor  generator  is  specially  well  suited,  closer  to  the  rotary 
transformer  in  cost  and  efficiency  than  would  be  supposed  off- 
hand, and  unique  in  the  complete  independence  of  its  working 
circuits. 

Practice  in  this  line  of  operations  has  not  yet  settled  into 
fixed  directions,  and  is  not  likely  so  to  do  just  at  present. 
Each  plant  nmst  therefore  be  considered  by  itself  and  treated 
symtomatically. 

American  usage  is  at  present  tending  strongly  toward  the 
rotary  converter,  on  account  of  its  ready  adaptation  to  railway, 
service,  but,  in  view  of  the  work  that  has  been  done  on  alternat- 
ing motors  for  such  service,  it  is  an  open  question  how  far 
current  reorganization  will  be  generally  necessary  in  the  future, 
although  just  now  it  is  of  very  great  practical  importance. 

As  the  price  of  copper  rises,  the  use  of  current  reorganizers 
becomes  more  and  more  important  in  railway  work,  and  for 
this  particular  use  the  rotary  converter  is  generally  chosen. 

There  should  be  mentioned  here  some  curious  and  valuable 
devices  for  obtaining  rectified  alternating  currents,  based 
upon  the  phenomena  of  polarization. 

Obviously,  if  one  could  find  a  conductor  which  would  let  pass 
cuiTents  in  one  direction,  and  block  those  in  the  other,  the 
result  of  putting  it  in  an  alternating  circuit  would  be  that  all 
the  current  impulses  in  one  direction  would  be  suppressed,  so 
that  the  resulting  current  would  be  a  series  of  separated  half- 
waves  of  the  same  polarity.  It  would  be  as  if  in  Fig.  166  all 
the  half-waves  above  the  base  line  were  erased.     Now  such  a 


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804  ELECTRIC  TRANSMISSION  OF  POWER, 

conductor  is  actually  obtainable  in  certain  electrolytic  cells  in 
which  a  counter  electromotive  force  or  severe  polarization 
resistance  impedes  current  flowing  in  a  particular  direction. 
Under  favorable  circumstances  the  selective  action  is  quite 
complete,  so  that  the  alternating  current  becomes  unidirec- 
tional. Fig.  179  shows  the  current  curve  for  a  complete 
cycle  as  modified  by  electrolytic  rectification.  The  positive 
half  of  the  wave  is  practically  wiped  out  of  existence.  The 
efficiency  of  these  electrolytic  devices  as  regards  the  energy 
rectified  is  quite  low,  and  most  of  the  apparatus  constructed 
has  been  upon  a  very  small  scale,  but  there  are  certain  purposes, 
like  energizing  induction  coils,  for  which  it  may  occasionally 
be  of  service.     It  is  given  place  here  more  on  account  of  its 


Fro.  179. 

general  interest  than  for  any  practical  value.  It  works  best, 
like  most  other  rectifying  devices,  at  low  frequencies. 

The  latest  and  in  some  respects  the  most  interesting  device 
for  obtaining  continuous  currents  from  an  alternating  source, 
is  the  vapor,  or  mercury  arc  converter.  Its  action  depends 
on  the  mechanism  of  current  flow  in  the  electric  arc.  As  is 
well  kno^Ti,  the  current  is  carried  across  the  space  between  the 
terminals  of  an  electric  arc  by  a  blast  of  vapor  streaming  from 
the  negative  to  the  positive  electrode.  An  arc  cannot  start 
until  this  stream  has  been  established,  for  which  reason  arcs 
are  generally  started  by  touching  the  electrodes  momentarily 
together.  For  the  same  reason  on  a  low  frequency  alternating 
circuit,  or  generally  unless  a  considerable  mass  of  conducting 
vapor  lingers  between  the  poles,  the  arc  readily  goes  out,  since, 
granted  that  the  arc  is  struck  at  all,  the  negative  stream  dies 
with  the  pulse  of  current  that  produced  it,  and  the  following 
alternation  can  only  get  through  by  starting  a  new  stream 
from  the  other  electrode  as  negative. 

Now,  the  arc  formed  about  a  mercury  negative  pole  in  vacuo 


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CURRENT  REORGANIZERS. 


806 


has  this  remarkable  property,  that,  while  once  started  the 
stream  can  be  maintained  by  a  few  volts,  it  takes  many  thou- 
sand volts  to  initiate  or  to  reestablish  the  stream  over  any 
material  gap.  Hence,  if  the  stream  is  once  started  it  can  be 
kept  in  action  continuously  by  a  rather  low  voltage  current, 
but  can  be  reversed  only  by  an  enormous  E.  M.  F.  in  the 
opposite  direction. 

If,  however,  the  original  negative  stream  can  be  kept  going 


Po«j(Sto£l«({tcod«« 


VttXLy^JSuMH  rode 


/WVVVWWsA/^ 
a 


A»<XSapplf  — 
FlO.  180. 


it  will  transmit  freely  current  impulses  in  the  original  direc- 
tion while  reverse  impulses  will  lack  the  potential  required  to 
reverse  the  stream.  Upon  this  property  of  the  mercury  arc 
the  vapor  converter  is  based,  and  the  essential  feature  of  its 
operation  is  the  preservation  of  the  negative  stream  by  send- 
ing overlapping  impulses,  so  that  once  started  the  original 
stream  shall  not  die  out.  The  extremely  ingenious  method 
of  doing  this  is  shown  in  Fig.  180.  Here  A  is  an  exhausted 
bulb  8  or  10  inches  in  diameter,  containing  two  positive  elec- 
trodes side  by  side,   and   a  mercury  negative  electrode  D. 


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806 


ELECTRIC  TRANSMISSION  OF  POWER. 


At  B  is  a  fairly  stiff  reactance.  The  two  positives  are  connected 
to  the  terminals  of  an  auto-converter  C  and  its  middle  point 
is  connected  to  B  through  the  proposed  d.  c.  circuit. 

The  apparatus  is  started  by  tipping  the  bulb  until  a  supple- 
mentary mercury  positive  touches  the  negative  and  as  the 
bulb  is  tipped  back  the  negative  stream  starts. 

Let  us  say  that  the  current  let  through  is  via  the  right  hand 
electrode.  Owing  to  the  reactance  B  the  current,  lagging, 
persists  until  the  E.  M.  F.  rising  in  the  left  hand  connections 


FIO.  181. 

has  had  time  to  start  via  the  same  negative  stream,  a  current 
through  the  other  positive  electrode.  Positive  electrodes 
virtually  in  the  same  negative  vapor  blast  can  thus  exchange 
work  freely,  provided  the  blast  be  not  interrupted. 

The  two  sides  of  the  circuits  thus  keep  up  the  interchange, 
working  alternately,  but  utilizing  as  \vill  be  seen  from  the 
consecutive  directions  of  flow,  both  sets  of  alternations.  By 
this  same  cause  the  effective  E.  M.  F.  of  the  rectified  current 
is  something  less  than  half  the  nominal  a.  c.  voltage  applied 
to  the  apparatus  as  a  whole.  By  a  stroboscopic  examination 
the  sequence  of  the  operations  can  be  very  beautifully  seen. 


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CURRENT  REORGANIZERS.  307 

Two-phase  or  three-phase  currents  can  be  made  operative  in 
a  very  similar  manner  so  that  the  process  is  a  general  one. 
It  can  be  made  operative  at  any  commercial  frequency. 

Fig.  181  shows  the  constant  current  form  of  the  same  device. 
The  letters  have  the  same  significance  although  the  electrode 
tube  is  of  different  shape,  and  the  coil  C  is  here  the  secondary 
of  a  constant  current  transformer.  The  resulting  current  from 
the  vapor  converter  is  evidently  not  uniform  but  somewhat 
pulsatory  as  if  received  from  a  dynamo  having  very  few  seg- 
ments in  the  commutator. 

Fig.  182  from  an  oscillograph  record  *  of  the  current  form 


Fio.  182. 

derived  from  the  constant  current  converter  like  Fig.  181, 
shows  the  facts  in  the  case  admirably. 

The  efficiency  of  such  apparatus  is  high.  There  is  a  small 
back  E.  M.  F.  of  about  15  volts  to  overcome,  the  ohmic  and 
hysteretic  loss  in  the  transformer  and  reactance,  and  some 
heating  of  the  c<jnverter  tube.  The  higher  the  voltage  applied 
to  the  tube  the  less  current  for  a  given  energy  and  the  better 
the  efficiency,  and  the  voltage  may  be  anything  that  will  not 
strike  a  reverse  arc  in  the  tube.  At  current  of  a  few  amperes 
the  working  a.  c.  voltage  may  even  be  25,000  volts.  At  mod- 
erate voltages  the  back  E.  M.  F.  is  more  important  and  the 
current  rises  for  the  same  energy  so  as  to  sooner  reach  the 
heat  endurance  of  the  tube. 

The  constant  potential  form  is  now  commercially  avail- 
able in  moderate  capacities,  say  up  to  25  or  30  KW  at  115  to 
120  volts,  the  efficiency  being  about  75  to  80  per  cent.  These 
converters  are  designed  for  charging  storage  batteries  and 
*  Steinmetz,  tr.  A.  L  E.  E.  June,  1906. 


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308  ELECTRIC  TRANSMISSION  OF  POWER, 

similar  light  work.  The  constant  current  form  is  beginning 
to  be  used  for  arc  lights,  giving  d.  c.  arcs  off  an  a.  c.  circuit, 
using  d.  c.  voltages  up  to  4,000  or  5,000  volts.  The  efficiency 
of  such  sets  is  probably  between  80  and  90  per  cent,  and  the 
power  factor  is  reported  to  be  .90  or  better. 

The  apparatus  is  very  beautiful  in  principle,  and  has  thus 
far  developed  no  serious  operative  defects.  Its  life  is  somewhat 
uncertain  and  a  good  deal  of  experimenting  is  still  needed  to 
bring  it  into  standard  form,  but  it  is  altogether  very  promising. 
Whether  it  is  to  be  available  for  large  powers  remains  to  be 
seen,  but  it  is  certain  to  find  a  wide  commercial  use  so  soon 
as  it  has  been  far  enough  standardized  to  enable  the  price  to  be 
brought  down  to  a  manufacturing  basis.  At  present  the 
figures  are  too  near  those  charged  for  motor  generators  to 
encourage  any  widespread  enthusiasm. 


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CHAPTER  VIII. 

ENGINES    AND    BOILERS. 

Mechanisms  that  constitute  the  link  between  natural  sources 
of  energy  and  mechanical  power  are  called  prime  movers.  So 
far  as  the  electrical  transmission  of  energy  is  concerned,  but 
two  classes  of  prime  movers,  steam  engines  and  water-wheels, 
have  to  be  seriously  considered.  All  others  sink  into  insigni- 
ficance or  are  limited  to  special  and  rarely-occurring  cases. 
When  power  is  transmitted  electrically  over  considerable  dis- 
tances the  prime  mover  is  usually  a  water-wheel,  since,  as  yet, 
the  transmission  of  power  from  coal  fields  has  been  hardly 
more  than  begun,  although  when  long  electrical  lines  be- 
come somewhat  more  familiar,  coal  may  become  a  frequent 
source  of  energy.  Where  the  distribution  of  power  from  a 
central  point  is  to  be  accomplished,  the  prime  mover  is  fre- 
quently a  steam  engine. 

The  general  principle  of  the  steam  engine  may  be  fairly 
supposed  to  be  somewhat  familiar  to  the  reader,  but  the  con- 
ditions of  economy  are  not  always  so  clearly  understood.  The 
source  of  power  in  an  engine  is  the  pressure  of  the  steam, 
which  must  be  utilized  as  fully  as  possible  to  get  anything  like 
efficient  working.  Since  the  pressure  is  in  direct  proportion 
to  the  temperature  in  any  gas,  the  proportion  of  the  total  pres- 
sure which  can  be  used  depends  on  the  original  temperature 
at  which  its  use  is  begun,  and  the  temperature  at  which  one 
ceases  to  use  it  and  rejects  it  together  with  all  the  energy  it 
then  possesses.  These  temperatures  are  not  to  be  reckoned 
from  the  ordinary  zero  of  a  thermometer,  but  from  the  so- 
called  absolute  zero.  This  is  that  point  from  which,  if  the 
temperature  of  a  gas  be  reckoned,  its  pressure  will  be  directly 
proportional  to  the  temperature.  It  is  461°  below  zero, 
Fahrenheit,  that  is,  493°  below  the  melting  point  of  ice.  It  is 
determined  by  the  consideration  that  any  gas  at  this  melting 
point  loses  ^i^  of  its  pressure  for  a  change  m  temperature  of 

309 


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310  ELECTRIC  TRANSMISSION  OF  POWER. 

one  degree,  hence,  if  it  could  be  cooled  down  493^,  would  lose 
its  pressure  and  would  have  given  up  all  of  its  energy.  Count- 
ing from  this  absolute  zero,  then,  one  can  utilize  that  part  of 
the  whole  energy  of  a  gas  which  lies  between  the  temperature 
at  which  the  gas  begins  to  work  and  that  at  which  it  ceases  to 
do  work.  In  other  words  the  efficiency  of  any  engine  operated 
by  gaseous  pressure  is: 

, 

in  which  T^  is  the  absolute  temperature  of  the  gas  when 
it  begins  to  do  work  in  the  engine,  and  7\  the  absolute  temper- 
ature at  which  its  work  ends.  In  practice,  T^  is  the  tem- 
perature of  the  steam  when  it  enters  the  cylinder,  and  T^ 
the  temperature  of  exhaust  or  condensation.  Steam  permits 
the  use  of  but  a  Umited  range  of  temperature  on  account 
of  the  temperature  at  which  it  liquefies,  and  bothers  us  by 
condensing  as  it  expands,  even  in  the  cylinder.  It  must  be 
remembered  that  while  we  are  limited  by  our  possible  range  of 
temperature  to  a  low  total  efficiency  in  any  heat  engine,  of  the 
energy  that  can  possibly  be  obtained  within  this  limitation, 
a  very  good  proportion  is  recovered  in  the  best  modern  engines 
—  from  one-half  to  three-fourths.  The  remainder  is  lost  in 
various  ways,  largely  through  radiation  of  heat  and  cylinder 
condensation.  Besides  these  thermal  losses  a  portion  of  the 
energy  utilized  is  wasted  in  friction  of  the  mechanism. 

From  these  considerations  we  may  derive  the  following 
general  principles  of  engine  efficiency: 

I.  The  steam  should  be  admitted  at  the  highest  pressure 
feasible  and  exhausted  at  the  lowest  pressure  possible. 

This  indicates  that  high  boiler  pressure  should  be  used,  and 
that  it  is  better  to  condense  the  steam  than  to  expel  it  into  the 
air,  as  by  condensing  most  of  the  atmospheric  pressure  can 
be  added  to  the  working  range  of  pressure  in  the  engine.  In 
the  next  place  it  is  evident  that  the  steam  should  be  sent  into 
the  engine  at  full  boiler  pressure,  and  finally  condensed  after 
expanding  and  yielding  up  its  pressure  as  completely  as 
possible. 

II.  Waste  of  heat  in  the  engine  should  be  stopped  as  far 


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ENGINES  AND  BOILERS.  311 

BS  possible.  This  means  checking  losses  from  the  cylinder 
by  radiation  and  conduction,  and  internal  loss  from  cylinder 
condensation.  The  first  principle  laid  down  has  for  its  ob- 
ject the  increase  of  the  possible  efficiency,  while  this  second 
principle  bears  on  the  securing  of  as  large  a  proportion  as 
possible  of  this  possible  efficiency.  It  requires  the  preven- 
tion of  escape  of  heat  externally  by  protecting  the  cylinder, 
and  incidentally  shows  the  advantage  of  high  pressure  and  high 
piston  speed  in  securing  as  much  work  as  possible  without  in- 
creasing the  size  of  the  working  parts,  and  hence  their  chance 
for  radiation.  On  the  other  hand,  it  indicates  the  danger  of 
working  with  too  great  a  range  of  temperature  in  the  cylinder 
thus  producing  cyUnder  condensation. 

III.  The  work  of  the  engine  should  be  the  maximum  practi- 
cable for  its  dimensions  and  use.  This  secures  high  mechan- 
ical efficiency  as  the  previous  principles  secure  high  thermal 
efficiency.  To  fulfill  this  condition  high  steam  pressure  and 
high  piston  speed  are  necessary,  and  the  latter  usually  means 
also  rather  high  rotative  speed.  The  importance,  too,  of  fine 
workmanship  in  the  hioving  parts  is  evident. 

It  will  be  realized  that  some  of  the  conditions  just  pointed 
out  are  mutually  incompatible  to  a  certain  extent.  Every- 
thing points,  however,  to  the  great  desirability  of  a  condens- 
ing engine,  worked  with  a  high  initial  steam  pressure  and 
great  piston  speed.  The  tendency  of  the  best  modem  prac- 
tice is  all  in  this  direction,  and  the  efficiency  of  engines  is  con- 
stantly improving.  The  greatest  advances  of  the  past  decade 
or  two  have  been  in  the  introduction  of  compound  engines. 
The  principle  here  involved  is  the  lessening  of  thermal 
losses  in  the  cylinder  by  avoiding  extremes  of  temperature 
between  the  initial  and  the  final  temperature  of  the  steam  ex- 
panded into  it.  Compound  engines  simply  divide  the  expan- 
sion of  the  steam  between  two  or  more  cylinders,  so  that  the 
temperature  range  hi  each  is  limited,  without  hmiting  the 
total  amount  of  expansion. 

Following  the  same  line  of  improvement,  triple  and  quad- 
ruple expansion  engines  are  becoming  rather  common,  although 
the  value  of  the  last  mentioned  is  somewhat  problematical  at 
present. 


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312  ELECTRIC  TRANSMISSION  OF  POWER, 

For  practical  purposes  steam  engines  may  be  classified  in 
terms  of  their  properties,  somewhat  as  follows  : 

First,  there  is  the  broad  distinction  between  condensing  and 
non-condensing  engines.  The  former  condense  the  exhausted 
steam  and  gain  thereby  a  large  proportion  of  the  atmospheric 
pressure  against  which  the  latter  class  is  obliged  to  do  work  in 
exhausting  the  steam.  Where  economy  of  operation  is  se- 
riously considered,  the  non-condensing  engine  has  no  place,  if 
water  for  condensation  is  obtainable. 

Each  of  these  classes  falls  naturally  into  subclasses,  depend- 
ing on  the  number  of  steps  into  which  the  expansion  is 
divided  —  simple,  compound,  triple  expansion,  etc.  Of  these 
the  first  may  now  and  then  be  desirable,  where  the  size  is  small 
and  coal  very  cheap,  but  for  the  general  distribution  of  energy 
the  last  two  are  more  generally  useful.  Furthermore,  each  of 
the  subclasses  mentioned  may  be  divided  into  two  genera,^ 
depending  on  the  nature  of  the  valve  motions  that  control  the 
admission  and  rejection  of  the  steam.  To  follow  out  the  first 
principle  of  economy  laid  down,  the  steam  must  be  admitted 
at  a  uniform  pressure  as  near  that  of  the  boiler  as  possible,  the 
admission  should  be  stopped  short  after  entrance  of  enough 
steam  for  the  work  of  the  stroke,  the  steam  allowed  to  expand 
the  required  amount,  and  then  rejected  completely  at  the  lowest 
possible  pressure.  The  admission  valves  should  therefore 
open  wide  and  very  rapidly,  let  in  the  steam  for  such  part  of 
the  stroke  as  is  necessary,  and  then  as  promptly  close.  The 
exhaust  valves  should  open  quickly  and  wide  when  the  expan- 
sion is  complete,  and  stay  open  imtil  nearly  the  end  of  the 
stroke,  closing  just  soon  enough  to  cushi(m  the  piston  at  the 
end  of  its  stroke.  In  proportion  to  the  completeness  with 
which  these  conditions  are  met,  the  use  of  the  steam  will  be 
economical  or  wasteful.  The  two  genera  of  engines  referred  to 
are  those  in  which  the  motions  of  the  admission  and  exhaust 
valves  are  independent  of  each  other  or  dependent.  Fig.  183 
shows  in  section  the  cylinder  and  valves  of  an  mdependent 
valve  engine,  Corliss  type.  The  arrows  show  the  flow  of  the 
steam.  The  admission  valve  on  the  head  end  of  the  cylinder 
has  just  been  opened,  as  also  has  the  exhaust  valve  on  the 
crank  end. 


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ENGINES  AND  BOILERS. 


313 


The  essential  point  of  the  mechanism  is  that  the  admission 
valves  open  and  close  at  whatever  time  is  determined  by  the 
action  of  the  governor  without  in  the  least  affecting  the  work- 
ing of  the  exhaust  valves.  In  the  Corliss  valve  gear  the 
steam  valves  are  closed  by  gravity,  or  by  a  vacuum  pot,  and 
are  opened  by  catches  moved  by  an  eccentric  rod,  and  released 
at  a  point  determined  by  the  governor,  which  thus  varies  the 
point  of  cut-off  according  to  the  load.  Ordinarily  the  admis- 
sion of  steam  is  thus  cut  off  in  a  simple  engine  at  full  load 
after  the  piston  has  traversed  from  one-fifth  to  one-quarter  of 
its  stroke,  according  to  the  pressure  of  the  steam.  If  the  cut- 
off is  too  late  in  the  stroke,  there  is  not  sufficient  expansion  of 


Fio.  183. 


the  steam ;  if  too  early  the  steam  is  partially  condensed  by  too 
great  expansion.  For  every  initial  pressure  of  steam  there  is  a 
particular  degree  of  expansion  which  gives  the  best  results  in  a 
given  engine. 

Fig.  184  shows  the  valve  motion  of  one  of  the  best  of  the 
dependent  valve  genus.  Steam  is  just  being  admitted  at  the 
head  end  both  around  the  shoulder  of  the  hollow  piston  valve 
and  through  the  ports  at  the  other  end  of  the  valve  via  the 
interior  space.  At  the  crank  end  the  exhaust  port  has  just 
been  fully  opened.  It  will  be  seen  that  any  change  in  the 
conditions  of  admission  also  involves  a  change  in  the  condi- 
tions of  exhaust,  and  although  some  variation  may  take  place 
in  the  latter  without  serious  result  on  the  economy,  simplicity 


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314 


ELECTRIC  TRANSMISSION  OF  POWER. 


in  the  valve  gear  has  been  gained  at  a  certain  sacrifice  of 
efficiency  in  using  the  st^am.  Both  independent  and  depen- 
dent valve  engines  have  many  species  differing  widely  in 
mechanism,  but  retaining  the  same  fundamental  difference. 
Of  the  two  genera,  the  independent  valve  engine  has  the 
material  advantage  in  efficiency,  and  under  similar  conditions 
of  pressure,  capacity,  and  piston  speed  consumes  from  10  to  20 
per  cent  less  steam  for  the  same  effective  power.     It  there- 


FlO.  184. 


fore  is  generally  employed,  in  spite  of  somewhat  greater  first 
cost,  for  all  large  work,  often  in  the  compound  or  triple  ex- 
pansion form.  Except  in  small  powers,  or  for  exceptionally 
high  speed,  the  dependent  valve  engine  has  few  advantages, 
and  in  the  generation  of  power  on  a  large  scale,  such  as  for 
the  most  part  concerns  us  in  electrical  transmission  work,  it 
hardly  has  an  important  place. 

It  must  not  be  supposed  that  between  the  various  sorts  of 
engines  mentioned  there  are  hard  and  fast  lines.  In  the 
economical  use  of  steam  a  very  large  non-condensing  engine 


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ENGINES  AND  BOILERS.  815 

may  surpass  a  smaller  condensing  one,  or  a  fast  rimning 
dependent  valve  engine,  a  very  slow  running  one  with  inde- 
pendent valves.  Broadly,  however,  we  may  lay  down  the 
following  propositions  concerning  engines  of  similar  capacity: 

I.  Condensing  engines  will  always  furnish  power  more 
economically  than  non-condensing  ones.  This  is  particularly 
true  at  less  than  fiJl  load,  since  the  loss  of  the  atmospheric 
pressure  may  be  taken  as  a  constant  source  of  inefficiency, 
which,  like  mechanical  friction,  is  very  serious  at  low  loads. 
For  example,  a  triple  expansion  engine  working  at  one-(juarter 
load  in  indicated  HP,  will  be  likely  to  have  its  ccmsumption  of 
steam  per  IHP,  increased  from  15  to  25  per  cent  above  the  con- 
sumption per  IHP  at  full  load;  while  worked  non-condensing, 
the  increase  would  be  from  50  to  100  per  cent.  Hence,  for 
electrical  working  where  light  loads  are  frequent,  condensing 
engines  are  an  enormous  advantage.  With  simple  or  compound 
engines  the  same  general  rule  holds  good  as  for  triple-expan- 
sion engines,  ^vith  the  additional  point  that  light  loads  affect 
their  economy  even  more,  when  worked  non-condensing.  It 
must  be  borne  in  mind  that  if  any  engine  is  to  do  its  best  under 
varying  loads,  its  valve  gear  and  working  pressure  must  be 
arranged  with  this  in  mind,  else  the  advantage  of  high  expan- 
sion and  condensing  may  be  thrown  away.  It  is  frecjuently 
said  that  triple  expansion  engines  do  not  give  good  results  in 
electric  railway  work.  When  this  is  the  case  there  has  been 
improper  adjustment  of  engine  to  load. 

II.  Among  engines  having  the  same  class  of  valve  gear, 
compound  engines  give  better  economy  than  simple  ones,  and 
triple  expansion  better  than  compound.  This  is  true  irre- 
spective of  the  nature  of  the  load,  supposing  each  engine  to  be 
suitably  adjusted  to  the  work  it  has  to  do.  In  rare  cases,  owing 
to  exceedingly  cheap  fuel  and  short  working  hours,  it  may  hap- 
pen that  the  advantage  of  a  triple  expansion  engine  over  a 
compound  in  economy  of  coal  may  be  more  than  offset  by 
increased  interest  on  investment,  but  at  the  present  cost  of 
engines  and  boilers,  this  could  not  well  occur  unless  in  the 
case  of  burning  culm  or  poor  coal  obtained  at  a  nominal 
price. 

III.  As  regards  speed  of  engines,  there  is  always  advantage 


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816  ELECTRIC  TRANSMISSION  OF  POWER. 

in  high  piston  speed  both  as  respects  first  cost  and  mechanical 
efficiency.  So  far  as  the  economical  use  of  steam  goes,  speed 
makes  little  difference  save  as  it  sometimes  involves  a  change 
in  the  valve  gear.  Most  high-speed  engines  have  valve  gear 
of  the  dependent  sort,  which  puts  them  at  a  disadvantage 
except  in  so  far  as  lessened  cylinder  condensation  and  friction 
may  offset  the  losses  due  to  less  efficient  distribution  of  the 
steam.  But  the  best  dependent  valve  engine  is  uniformly  less 
economical  than  the  best  independent  valve  engine  of  the 
same  class  and  subclass.  Even  the  lessened  friction  of  the 
small  high-speed  pistons  does  not  offset  this  difference  in 
intrinsic  economy. 

As  regards  actual  economy  in  the  st^am  consumption,  the 
size  of  engine  has  a  powerful  though  somewhat  indeterminate 
influence.  Even  at  full  load,  simple  non-condensing  dependent 
valve  engines  of  moderate  size  require  from  30  to  40  lbs. 
of  steam  per  indicated  horse-power  hour.  Only  in  very  large 
engines,  such  as  locomotives,  and  specially  fast  running  engines 
such  as  the  Willans»  does  the  steam  consumption  of  these 
dependent  valve  engines  fall  below  30  lbs.,  and  not  very  often 
even  in  these  cases.  Worked  condensing  the  same  machines 
use  from  20  lbs.,  hi  exceedingly  favorable  cases,  to  25  or  30  lbs. 
more  commonly. 

Independent  valve  engines,  simple  and  non-condensing,  will 
give  the  indicated  HPH  on  25  to  30  lbs.  of  steam,  occasion- 
ally on  as  little  as  22  to  23  lbs.  With  the  advantage  of  con- 
densation these  figures  may  be  reduced  to  say  18  to  25  lbs., 
the  former  figure  being  somewhat  exceptional  and  probably 
very  rarely  attained  in  practice. 

Passing  now  to  compoimd  non-condensing  engines,  the  effect 
of  compounding  on  efficiency  is  about  the  same  as  that  of  con- 
densing. Ordinary  dependent  valve  engines  of  compound 
construction  require  from  20  to  25  or  30  lbs.  of  steam  per 
IHP  hour.  The  former  result  is  very  exceptional,  and  seldom 
or  never  reached  in  practice,  while  the  last  mentioned  would 
be  considered  rather  high.  Independent  valve  compound 
engines  are  so  seldom  worked  non-condensing,  that  the  data  of 
their  performance  are  rather  meagre;  18  to  25  lbs.  of  steam 
is  about  the  usual  amount,  however. 


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ENGINES  AND  BOILERS.  317 

When  condensation  is  employed,  on  the  other  hand,  the 
dependent  valve  engines  are  in  rather  infrequent  use.  When 
ihe  need  for  economy  is  so  felt  as  to  lead  to  the  use  of  com- 
pound engines,  it  also  leads  to  the  use  of  economical  valve 
gear.  The  steam  consumption  of  dependent  valve  compound 
condensing  engines  is  quite  well  known,  however,  and  is 
usually  from  16  lo  24  lbs.  per  IHP  hour.  The  first  mentioned 
figure  is  rarely  reached,  and  only  in  special  types  of  engines. 

Plenty  of  tests  on  compound  condensing  engines  with  inde- 
pendent valves  are  available;  14  to  20  lbs.  of  steam  covers 
the  majority  of  results.  Occasional  tests  run  down  to  and 
even  below  12  and  as  high  as  22  lbs. 

It  is  noticeable  that  in  compoimd  engines  the  difference 
between  dependent  and  independent  valve  gear  is  l6ss  than 
with  simple  engines.  This  is  due  to  a  variety  of  causes.  The 
larger  range  of  expansion  used  in  compound  engines  tends  to 
lessen  the  deleterious  effects  of  moderate  variations  in  the 
distribution  of  the  steam,  and  besides,  the  valve  gear  of  com- 
pound engines  is  not  infrequently  composite,  the  high-pres- 
sure cylinder  having  independent  valves  and  the  low-pressure 
cylinder  dependent  ones. 

The  same  arrangement  is  often  used  in  triple  expansion 
engines,  so  that,  in  conjunction  with  the  condition  before 
mentioned,  it  is  usually  true  that  the  economy  of  dependent 
valve  triple  expansion  engines  is  much  nearer  that  of  indepen- 
dent valve  ones  than  would  be  at  first  supposed.  Without 
condensing,  a  dependent  valve  triple  expansion  engine  may  be 
expected  to  require  from  19  to  27  lbs.  of  steam  per  IHP  hour. 
With  condensation  such  engines  perform  much  better,  the  steam 
consumption  being  reduced  to  14  to  20  lbs. 

Nearly  all  triple  expansion  engines,  however,  are  built  with 
independent  valves,  at  least  in  part,  the  intention  being  to 
secure  the  most  economical  performance  possible.  Under 
favorable  conditions  their  steam  consumption  runs  as  low  as 
12  lbs.  per  IHP  hour,  and  seldom  rises  above  18  lbs.  In  a 
few  exceptional  cases  the  record  has  been  reduced  below 
12  lbs.,  but  such  results  cannot  often  be  expected.  Any- 
thing under  13  lbs.  of  steam  per  HP  is  good  practice  for 
running  conditions. 


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ai8 


ELECTRIC  TRANSMISSION  OF  POWER. 


All  the  figures  given  refer  in  the  main  to  good  sized  engines 
of  at  least  200  HP  and  over,  operated  at  full  load  and  at 
favorable  ratios  of  expansion.  It  must  be  clearly  understood 
that  there  is  for  each  steam  pressure  a  particular  ratio  of 
expansion  which  will  give  the  most  economical  result  —  less  ex- 
pansion than  this  rejects  the  steam  at  too  high  a  temperature; 
more,  causes  loss  by  condensation,  etc.  Compound  and  triple 
expansion  engines  permit  greater  expansion  of  the  steam  with- 
out loss  of  economy,  hence  allow  higher  steam  pressure  and  a 
greater  temperature  range  —  hence  higher  thermal  efficiency. 
Good  practice  indicates  that  for  simple  engines  the  boiler 
pressure  should  be  not  less  than  90  to  100  lbs.  per  square 
inch,  for  compound  engines  not  less  than  120  to  150,  and  for 
triple  expansion  engines  not  less  than  140  to  150,  and  thence 
up  to  175  or  200  lbs. 

We  may  gather  the  facts  regarding  steam  consumption  into 
tabular  form  somewhat  as  follows: 


Kind  of  Engine. 

Steam  per  I  HP. 
General  Range. 

Steam  per  IHP. 
Working  Average. 

Simple,  non-condensing  dep  v 

30-40 
25-30 
20-30 
18-25 
20-28 
18-26 
10-24 
14-20 
14-20 
12-18 
12-14 

38 

Simple,  non-condensing  indep.  v 

Simple,  condensing  dep.  v 

28 
25 

Simple,  condensing  indep.  v 

21 

Compound,  non-condensing,  dep.  v 

Compound,  non-condensing  indep.  v 

Compound,  condensing  dep.  v 

24 
22 
20 

(/Ompound,  condensing  indep.  v 

17 

Triple,  condensing  dep.  v 

17 

Triple,  condensing  indep.  v 

14 

Triple  large,  condensing  indep.  v 

13 

The  engines  considered  are  supposed  to  be  of  good  size  — 
say  200  to  500  IIP,  ami  to  be  worked  steadily  at  or  near  full 
load.  The  figures  given  as  working  average  are  such  as  may 
be  safely  counted  on  with  good  engines,  kept  in  the  best 
working  condition,  and  operated  at  at  least  the  boiler  pres- 
sures indicated.  The  steam  is  supposed  to  be  practically  dry 
and  the  piping  so  protected  as  to  lose  little  by  condensation. 
These  results  are  such  as  may  regularly  be  obtained  in  prac- 
tice, and  indeed  it  is  not  uncommon  to  find  them  excelled. 
Compound  condensing  engines  of  large  size  not  infrequently 


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ENGINES  AND  BOILERS.  319 

work  down  to  13  lbs.  of  steam,  and  triple  expansion  con- 
densing engines  down  to  12  lbs.,  which  result  will  be  guar- 
anteed by  most  responsible  builders. 

Unfortunately,  engines  employed  for  electrical  work  are  com- 
paratively seldom  kept  at  uniform  full  load.  Furthermore, 
they  are  subject  to  all  sorts  of  variations  of  load.  In  electric 
railway  service  there  arc  sudden  changes  from  light  loads  to 
very  heavy  ones,  while  in  electric  lighting  there  is  generally  a 
gradual  increase  to  the  maximum  load,  which  continues  an 
hour  or  two,  followed  by  a  rather  gradual  decrease.  Thase 
variations  affect  the  economy  of  the  engines  unfavorably  —  at 
certain  loads  there  is  not  enough  expansion,  at  others  decidedly 
too  much.  The  variations  in  economy  are  largely  controlled 
by  the  proportioning  of  the  engine  to  its  work.  To  say  that 
an  engine  is  of  500  HP  means  little  unless  the  statement  be 
coupled  with  a  definite  explanation  of  the  circumstances.  If 
that  output  is  obtained  by  admitting  steam  for  half  the  stroke, 
the  engine  will  work  at  500  HP  very  uneconomically,  sup- 
posing a  simple  engine  to  be  under  consideration.  Its  point 
of  maximum  economy  may  be  perhaps  300  HP.  On  the  other 
hand,  500  HP  may  be  given  when  cutting  off  the  steam  at  one- 
fifth  stroke.  In  this  case  the  engine  will  be  working  near  its 
point  of  maximum  economy,  and  at  300  HP  will  require  much 
more  steam  per  IHP.  It  could  give  probably  600  to  700  HP 
at  a  longer  cut-off,  and  is  really  a  much  more  powerful  engine 
than  the  first.  For  uniformity  it  is  better  to  rate  an  engine  at 
the  HP  of  maximum  economy,  whatever  the  real  load  may  be. 
The  relation  of  load  to  economy  is  well  shown  in  the  curves  of 
Fig.   185. 

Curves  1,  2,  4,  and  5,  are  of  engines  so  rated  as  to  have 
their  maximum  economy  near  full  load.  Curve  3,  (m  the 
other  hand,  is  from  an  engine  inteiided  to  give  its  highest 
economy  at  about  three-cjuarters  load.  For  very  variable 
output  this  is  the  preferable  arrangement,  while  for  large 
central  station  work,  when  the  number  of  units  is  large  enough 
to  permit  loading  fully  all  that  are  running  at  any  one  time, 
it  is  better  to  have  each  unit  give  its  veiy  best  economy  near 
full  load  and  to  vary  the  number  of  units  according  to  the  re- 
quirements of  total  load. 


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ELECTRIC  TRANSMISSION  OF  POWER. 


For  electric  railway  service  under  ordinary  conditions,  it  is 
best  to  employ  an  engine  which  at  full  load  is  worked  to  a 
high  capacity,  and  hence  somewhat  uneconomically,  while  at 
lesser  loads,  which  more  nearly  correspond  with  the  average 
conditions,  its  economy  will  be  at  a  maximum.  For  electric 
lighting  service  it  is  preferable  to  have  the  point  of  maximum 


PER  CENT  LOAD  l.H.P. 
Fio.  185. 

economy  fall  more  nearly  at  full  load.  For  power  service, 
which  is  on  the  one  hand  more  uniform  than  railway  service,  and 
less  uniform  than  electric  lighting  work,  it  is  probably  best  to 
employ  an  engine  having  characteristics  between  those  just 
mentioned".  In  every  case  attention  must  be  paid  to  the 
character  of  the  load  as  regards  average  amount  and  con- 
stancy in  the  choice  of  an  appropriate  engine  for  the  work. 


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ENGINES  AND  BOILERS. 


821 


In  cases  where  the  variations  of  load  are  likely  to  be  very 
sudden,  great  mechanical  strength  of  all  the  moving  parts  is 
absolutely  necessary,  and  an  attempt  should  be  made  in  plan- 
ning the  power  station  to  arrange  the  engine  for  its  best  econ- 
omy at  average  load  as  nearly  as  this  can  be  predicted. 

With  care  in  planning  an  electric  power  station  the  engines 
can  be  made  to  give  an  exceedingly  good  performance,  much 


0^  «.?£  1.0 

FRQPORTION  TfiAT  ACTUAL  J.OAD  BEXH3  TO  RATED  POWER 

Fig.  186. 


better  than  was  considered  possible  a  few  years  ago.  Fig. 
186  shows  a  set  of  curves  from  the  experiments  of  Prof.  R.  C. 
Carpenter  giving  the  performance  of  engines  of  different  kinds 
over  a  wide  range  of  loads,  from  mere  friction  load  up  to  50 
per  cent  overload.  The  results  are  in  pounds  of  water 
evaporated  per  indicated  HPH.  The  immense  advantage  to 
be  gained  by  using  compound  and  triple  expansion  condensing 


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822  ELECTRIC  TRANSMISSION  OF  POWER. 

engines  appears  plainly  from  the  curves.  Another  conspicu- 
ous fact  is  the  great  economy  attained  by  such  engines  over  a 
wide  range  of  load.  It  is  a  common  fallacy  to  suppose  that 
while  compound  or  triple  expansion  condensing  engines  are  all 
well  enough  at  steady  load,  simple  engines  have  the  advantage 
if  the  load  varies  over  a  wide  range.  The  facts  in  the  case  as 
shown  in  Fig.  186  are  exactly  the  reverse:  not  only  do  the  high 
expansion  engines  have  the  advantage  of  the  simple  engines 
at  their  rated  loads,  but  at  all  loads,  and  particularly  light  ones. 
And  their  advantage  is  so  great  that  imder  any  ordinary  cir- 
cumstances the  use  of  a  simple  or  a  non-condensing  engine  for 
power  generation  is  wilful  waste  of  money.  If  the  saving  in 
first  cost  were  great  the  mistake  might  be  excusable,  but  the 
greater  amount  of  steam  required  for  nmning  simple  engines 
means  larger  boiler  capacity,  which  nearly  offsets  the  lower 
cost  of  engine.  For  example,  a  glance  at  Fig.  186  shows  that 
a  triple  expansion  condensing  engine  requires  only  half  the 
boiler  capacity  demanded  by  a  non-condensing  automatic  engine 
for  the  same  output.  In  other  words,  if  the  former  requires 
500  HP  in  boilers,  the  latter  will  need  1,000  HP  in  boilers  for 
exactly  the  same  service.  And  the  same  holds  true  of  the 
capacity  of  the  stack,  feed-pumps,  steam-piping,  water-piping, 
and,  to  a  certain  extent,  even  of  the  building,  so  that  it  is 
almost  always  poor  economy  to  buy  a  cheap  type  of  engine. 

The  greatest  improvement  in  economy  made  in  recent  years 
has  been  the  introduction  of  superheating  which  American 
engineers  have  been  somewhat  slow  in  adopting.  This  is  simply 
the  heating  of  the  steam  as  such  on  its  way  to  the  engine. 
The  steam  prior  to  use  is  passed  through  a  special  reheater, 
frequently  with  an  independent  furnace,  and  given  additional 
heat  energy,  the  working  temperature  being  thus  raised 
sometimes  to  600°  or  700°  F.  This  largely  increases  the 
range  of  working  temperature  possible  to  the  engine,  and  hence 
the  efficiency,  at  a  relatively  small  expense  for  extra  fuel. 

The  very  high  temperature  of  the  steam  compels  extra 
precautions  in  the  lubrication,  and  for  some  years  this  lubri- 
cation bug-a-boo  stood  in  the  way  of  substantial  progress. 
At  present  it  is  entirely  practicable  to  lubricate  the  cylinders 
successfully  even  up  to   the    figures   mentioned  above,  and 


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ENGINES  AND  BOILERS. 


323 


it  Ls  being  done  abroad  though  the  prejudice  in  this  country 
still  persists.  The  results  are  startling,  the  steam  consumption 
under  test  havmg  repeatedly  run  down  near 'and  even  below 
10  lbs.  of  steam  per  IHP  hour  in  compound  condensing  engines. 
The  result  of  a  recent  test  of  a  21  x  36  X  36-inch  mill  engine 
are  given  in  Fig.  187  and  represent  the  highest  efficiency  yet 
attained.  It  will  be  noted  that  under  the  test  conditions  with 
steam  superheated  to  720®  -  750®  F.  the  steam  consumption 
increased  for  the  heavy  loads  just  as  it  rises  for  overloads  in 
the  curves  of  Fig.  186.  The  fact  in  each  instance  merely 
implies  that  a  certain  amount  of  expansion  corresponds  to 


9.5 

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maximum  economy,  and  less  than  this  amount  injures  econ- 
omy although  it  increases  the  possible  output. 

Fig.  187  is  merely  an  extreme  instance  of  the  general  principle, 
due  to  starting  the  expansion  at  a  relatively  very  high  temper- 
atin-e.  There  is  no  doubt  whatever  of  the  practicability  of 
reducing  steam  expenditure  20  to  30  per  cent  below  that 
found  in  the  best  current  practice  by  an  amount  of  super- 
heating applicable  without  any  considerable  difficulty.  Super- 
heaters have  already  been  introduced  here  as  auxiliaries  to 
steam  turbines  with  pretty  good  effect,  but  have  not  yet  come 
into  more  than  occasional  use  for  general  purposes,  and  even 
so  are  very  rarely  worked  for  what  they  are  really  worth 

Large  gas  engines  are  beginning  to  come  into  use  as  prime 
movers  for  electrical  purposes,  and  one  such  plant  of  12,000 
KW  capacity  is  just  being  installed  in  San  Francisco.  The 
gas  engine  in  large  sizes  shows  very  great  thermal  efficiency, 
giving  the  brake  HP  hour  on  the  thermal  equivalent  of  1  lb. 


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S24  ELECTRIC  TRANSMISSION  OP  POWER. 

of  coal  or  even  less,  and  is  to-day  becoming  a  formidable  com- 
petitor of  steam  engines  for  many  purposes.  Working  as  it 
does  from  a  very  high  initial  temperature,  its  theoretical  claim 
to  efficiency  is  valid  enough,  and  the  difficulties  of  lubrication 
at  the  temperature  involved  have  proved  less  serious  than 
was  first  supposed.  The  main  trouble  is  the  fact  that  ordi- 
narily only  every  fourth  stroke  is  a  working  stroke,  so  that  for 
a  given  number  of  impulses  per  revolution  of  the  fly-wheel  the 
gas  engine  becomes  far  more  heavy  and  complex  than  the 
steam  engine.  Nevertheless,  the  gain  in  fuel  economy  is  so 
valuable  that  the  incentive  to  use  gas  engines  is  great.  They 
are  usually  worked  in  the  large  sizes  with  natural  or  "producer" 
gas,  sometimes  with  gas  from  the  blast  furnaces  of  the  steel 
industry,  in  other  words  with  cheap  gas  unsuited  for  illumi- 
nating purposes,  and  have  the  merit  of  being  very  quickly 
brought  into  action  when  required.  Difficulties  of  governing, 
once  serious,  have  now  been  in  great  measure  eliminated. 

Many  blunders  are  made  by  being  too  hasty  in  buying 
engines  for  electric  service,  and  not  sufficiently  studying  the 
problem.  For  uniform  loads  the  selection  of  the  engines  can 
be  made  easily.  For  variable  loads  it  requires  great  astute- 
ness and  experience,  nor  is  it  safe  to  argue  from  experience 
based  on  other  kinds  of  variable  service.  No  engines  can  be 
subject  to  greater  variations  of  load  than  are  met  in  marine 
engines  driving  a  ship  in  a  high  sea.  If  the  screw  rises  from 
the  water  the  whole  load  is  thrown  off,  and  resumed  again  with 
terrible  violence  when  the  screw  is  submerged.  Nevertheless 
an  engine,  which  is  so  arranged  as  to  perform  well  luider  these 
trying  circumstances,  might  perform  badly  when  put  on 
electric  railway  or  power  service,  not  because  of  its  inability 
to  stand  the  far  less  severe  changes  of  load,  but  for  the  reason 
that  the  average  load  would  be  much  further  from  its  full 
capacity  than  in  the  case  of  marine  practice.  For  large  rail- 
way and  power  service  it  is  best  to  use  direct  connected  units, 
for  the  sake  of  compactness  and  economy.  If  a  station  is  of 
sufficient  magnitude  to  employ  four  or  five  500  HP  engines, 
direct  connecting  is  advisable  in  nearly  every  case. 

It  has  been  said  that  such  a  plant  has  a  lack  of  flexibility 
that  is  dangerous  in  case  of  sudden  and  great  variations  of 


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ENGINES  AND  BOILERS.  825 

load.  This  ia  not  true  if  the  engines  have  been  intelligently 
proportioned  for  the  work  they  have  to  do,  although  in  some 
cases  there  has  been  trouble  due  to  the  fact  that  the  engines 
were  ill-fitted  to  operate  successfully  under  the  changes  of 
load  to  which  they  were  subjected.  As  a  matter  of  economy 
both  in  engines  and  dynamos,  it  is  desirable  to  work  direct 
coupled  plants  at  a  fairly  high  speed.  There  is  no  need  of  ex- 
aggerating the  size  of  both  engine  and  dynamo  for  the  sake  of 
rimning  at  50  to  70  revolutions  per  minute,  when  equally  good 
engines  and  dynamos  of  smaller  size  and  less  weight  can  be 
obtained  by  running  at  90  to  120  revolutions  or  more.  Much 
of  the  unwieldiness  charged  against  large  direct  coupled  units 
has  been  the  result  of  yielding  to  the  importunities  of  some 
engine  builder  who  wanted  to  sell  a  very  large  machine,  and 
putting  in  an  engine  and  draamo  working  at  absurdly  and  un- 
necessarily low  speed. 

Electric  power  transmission,  with  a  steam  engine  as  the 
prime  mover,  is  most  likely  to  be  developed  in  the  direction  of 
very  large  plants,  to  which  these  remarks  apply  most  forcibly, 
particularly  as  in  order  to  make  transmission  of  power  from 
a  steam-operated  station  profitable,  it  is  necessary  to  seek  the 
very  highest  efficiency.  Apart  from  the  cost  and  inconvenience 
of  very  low  speed  luiits,  it  must  be  borne  in  mind  that  the 
mechanical  efficiency  of  large  low  speed  engines  with  heavy 
pistons  and  enormous  fly-wheels,  is  lower  than  that  of  those 
designed  for  more  reasonable  speeds,  which  gives  added  reason 
for  moderation  in  planning  direct  coupled  units. 

Throughout  the  design  of  a  power  station  the  probability  of 
light  loads  must  be  considered.  Not  only  does  this  have  an 
important  bearing  on  the  economy  of  the  engines,  but  it  influ- 
ences that  of  the  boilers  as  well.  The  cost  of  operation  de- 
pends on  the  coal  consumption,  and  this  in  turn  not  only  on 
the  amount  of  steam  that  must  be  produced,  but  on  the  effi- 
ciency of  its  production. 

There  is,  however,  no  classification  of  boilers  on  which  one 
can  safely  rest  in  judging  of  their  economy.  There  is  much 
more  difference  in  economy  between  a  carefully  fired  and  a 
badly  fired  boiler  of  the  same  kind,  than  there  is  between  the 
best  and  the  worst  type  of  boiler  in  ordinary  use.     Boilers  may 


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ELECTRIC  TRANSMISSION  OF  POWER. 


be  generally  divided  into  three  classes:  Shell  boilers,  in  which 
the  water  is  contained  in  a  plain  cylindrical  tank  heated 
on  the  outside;  tubular  boilers,  in  which  there  are  one  or 
many  tubes  running  lengthwise  of  the  boiler  shell,  and  serving 
as  channels  for  the  heated  gases  from  the  fire;  and  water-tube 
boilers,  in  which  the  water  is  contained  in  a  group  of  metallic 
tubes,  around  which  the  heat  of  the  fire  freely  plays.  Fig.  188 
shows  a  cross  section  through  the  furnace  of  a  bank  of  boilers 
of  the  first  class.  In  this  case,  three  shells  were  placed  over 
each  furnace,  commimicating  with  a  common  steam  drum. 
Each  shell  was  30^  in  diameter  and  30'  long.  Fig.  189  rep- 
resents one  of  the  many  forms  of  tubular  boiler.  In  this  the 
structure  is  vertical,  with  a  furnace  at  the  bottom,  and  the 


Fia.  188. 

tubes  are  numerous  and  rather  small,  giving  a  large  heating 
surface.  Tubular  boilers  are  very  often  arranged  horizontally, 
and  in  one  very  excellent  and  common  type  (return  tubular), 
the  flame  and  heated  gases  pass  horizontally  under  the  boiler 
shell  and  then  back  through  the  tubes  to  the  furnace  end 
and  thence  upward  into  the  stack.  A  typical  water-tube 
boiler  is  shown  in  Fig.  190.  Here  the  furnace  is  at  the  left 
of  the  cut  and  the  stack  at  the  right.  The  tubes  are  inclined  as 
is  usual  in  water-tube  boilers,  and  steam  space  is  secured  by 
the  drum  above.  Each  class  of  boiler  has  nearly  as  many 
modifications  as  there  are  makers,  most  of  them  being  with 
relation  to  the  arrangement  of  the  fire  with  respect  to  the 
boiler  proper. 

As  to  the  merits  of  the  different  classes,  opinions  differ  very 
widely.     It  is   clear  from  experience  that  the  simple  shell 


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827 


boiler  is  decidedly  inferior  to  either  of  the  others  in  econ- 
omy, in  spite  of  its  simplicity  and  cheapness.  Of  late  years  it 
has  been  the  fashion  to  employ  water-tube  boilers  under  all 


Fio.  180. 


sorts  of  conditions,  on  account  of  their  supposed  great  effi- 
ciency as  steam  producers,  safety,  and  compactness.  Purely 
experimental  runs  with  such  boilers  often  show  phenomenal 
efficiency,  but  teste  under  working  conditions  sometimes  r^* 


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828 


ELECTRIC  TRANSMISSION  OF  POWER, 


sidt  otherwise.  It  is  important  to  note  that  not  only  does 
skill  in  firing  produce  a  great  improvement  in  boiler  economy, 
but  that  by  influencing  the  firing  different  kinds  of  coal  give 
very  different  results  quite  independent  of  their  theoretical 
value  as  fuel.  The  thermal  value  of  coal,  or  other  solid  fuel, 
is  almost  directly  as  the  proportion  of  carbon  contained  in  it, 
and  for  comparative  purposes  boiler  tests  are  generally  re- 
duced to  evaporation  of  water  from  and  at  212°  F.  per  pound 
of  combustible  used,  i.e.,  per  pound  of  carbon.  However,  the 
firing  in  different  furnaces  is  differently  affected  by  changes  in 


mmmm?///i/////m//mm 


FlO.  190. 


fuel,  so  that  it  is  impossible  to  predict  by  tests  on  one  boiler 
what  a  similar  one  will  do  imder  other  conditions. 

Altogether,  the  subject  of  boiler  efficiency  is  a  difficult  and 
tangled  one,  since  the  conditions  are  constantly  changing, 
and  the  best  guide  is  fomid  in  the  general  result  of  a  long  series 
of  tests  rather  than  in  theories  of  combustion.  Forcing  the 
output  of  a  boiler  usually  injures  its  efficiency  by  compelling 
the  combustion  of  an  abnormal  amount  of  coal  for  the  grate 
surface  of  the  furnace.  It  follows  that  a  boiler  is  apt  to  be 
more  efficient  at  moderate  loads  than  at  very  high  ones.  In 
marine  practice,  boilers  may  sometimes  have  to  be  forced  to 
a  high  output  to  save  weight  and  space.     In  electric  stations 


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ENGINES  AND  BOILERS. 


829 


it  is  sometimes  better  to  force  the  boilers  at  the  hours  of  heavy 
load,  than  to  keep  a  relay  of  boilers  banked  in  readiness  for 
use,  but  except  for  this,  the  boilers,  like  the  rest  of  the  plant, 
should  be  worked  as  near  their  maximum  efficiency  as  possible. 
The  best  fuel  to  use  is  not  at  all  invariably  that  of  the 
highest  thermal  value,  in  fact  with  the  proper  furnace  a  grade 
of  coal  only  moderately  good  is  very  often  the  most  economic 
cal.  In  starting  a  steam  plant  of  any  kind  comparative  tests 
of  various  coals  should  generally  be  made,  and  are  more  than 
likely  to  pay  for  themselves  many  times  over.  In  absolute 
heating  value  various  kinds  of  fuel  compare  about  as  follows: 


Kind  of  Fuel. 


Heat  of  Combiution. 

Evaporatien. 

15,260 

15.8 

14,600 

16.0 

14,875 

14.9 

18,750 

14.2 

12,760 

18.2 

12,500 

18.0 

11,750 

12.2 

9,650 

10.0 

7,250 

7.6 

Best  anthracite . . 
Welsh  steam  cosd 

Pocahontas 

Cumberland 

Coke,  ordinary  . . 

Cape  Breton 

Lignite 

Peat,  dry 

Wood,  dry 


The  heat  of  combustion  is  per  pound  of  fuel,  and  is  given  in 
thermal  imits,  this  imit  being  the  heat  required  to  raise  1  lb. 
of  water  1^  F. 

The  evaporation  gives  the  poimds  of  water  which  can  be 
evaporated  from  and  at  212°  F.  by  the  complete  utilization 
of  the  annexed  heats  of  combustion.  In  other  words,  no  more 
than  15  lbs.  of  water  can  possibly  be  evaporated  by  1  lb. 
of  coal  of  the  thermal  value  of  14,500.  Extravagant  claims 
are  sometimes  made  for  patented  boilers  of  strange  and 
imusual  kinds,  so  it  is  well  to  bear  these  figures  in  mind  and 
to  remember  that  you  cannot  evaporate  more  water  than 
the  figures  indicate,  any  more  than  you  can  draw  a  gallon 
out  of  a  quart  bottle.  In  practice  coal  is  likely  to  fall  perhaps 
10  per  cent  below  the  thermal  values  given  above.  Good 
boilers  with  careful  firing  will  utilize  from  70  to  75  per  cent 
of  the  thermal  value  of  the  coal.  Occasional  experimental 
runs  may  give  slightly  higher  figures,  but  only  under  very 
exceptional  circumstances. 


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ELECTRIC  TRANSMISSION  OF  POWER. 


Now  as  to  actual  tests  under  boilers.  Examinations  of 
more  than  a  hundred  carefully  conducted  tests  by  various 
authorities  show  from  8  to  13  lbs.  of  water  evaporated 
from  and  at  212°  per  poimd  of  combustible.  As  average  good 
steam  coal  contains  from  8  to  15  per  cent  of  ash  and  mois- 
ture, these  results  correspond  to  from  7  to  11 J  lbs.  of  water 
per  pound  of  coal.  Now  and  then  a  single  test  gives  a 
result  a  few  hundredths  of  a  pound  better  than  13  lbs.  per 
•pound  of  combustible,  and  an  occasional  poor  boiler  shows  less 
than  8  lbs.  Generally  from  10  to  16  lbs.  of  coal  are  consumed 
per  hour  per  square  foot  of  grate  surface.  The  following 
table  gives  a  general  idea  of  the  results  of  boiler  tests,  good, 
bad,  and  indifferent. 


Kind  of  Boiler. 

Kind  of  Coal. 

Evaporation. 

Return  tubular 

Welsh  steam 

18.12 

Water-tube 

(  Bituminous,  8  parts  1 

pea  and  dust,  1  part  j 

Cumberland 

18.01 

Return  tubular 

12.47 

Vertical  tubular 

Cumberland 

12.29 

Return  tubular 

Cnmh^rlftTld 

12.07 

Return  tubular 

Cumberland 

12.03 

Return  tubular 

Anthracite 

11.68 

Marine 

Newcastle 

11.44 

Water-tube 

Anthracite 

11.81 

Water-tube 

Cumberland 

10.98 

Plain  tubular 

Anthracite 

10.88 

Water-tube 

CnvnY>p,rland    . .    . 

10.79 

Marine. , 

10.44 

Return  tubular 

Anthracite 

10.48 

Liooomotive 

Coke 

10.39 

Water-tube 

Anthracite 

10.00 

Return  tubular 

Anthracite 

9.55 

Cylinder 

Anthracite 

9.22 

Cylinder 

Cumberland 

8.74 

Cylinder 

Anthracite 

8.44 

The  evaporation  is  per  pound  of  combustible.  The  most 
striking  feature  of  this  table  is  the  small  difference  in  efficiency 
between  the  various  kinds  of  boiler.  Putting  aside  the  cylin- 
drical shell  boilers,  which  are  distinctly  inferior  to  the  others, 
it  appears  that  in  other  types  of  boiler  there  is  little  to  choose 
on  the  score  of  economy  alone.  The  difference  between  the 
better  and  worse  boilers  of  each  class,  due  to  difference  of 
design,  condition,  and  firing,  is  much  greater  tlwi  th^  differ- 


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ENGINES  AND  BOILERS,  881 

ence  between  any  two  classes.  Even  the  same  boUer  with 
different  fuels,  firing,  or  when  in  different  condition,  may  give 
evaporative  results  var3dng  by  30  per  cent.  Economy  de- 
pends vastly  more  on  careful  firing  and  proper  proportion- 
ing of  the  grate  and  heating  surfaces  to  the  fuel  used,  than 
upon  the  kind  of  boiler.  In  fact,  judging  from  all  the  available 
tests,  the  differences  between  various  types  of  boiler  when 
properly  proportioned  are  quite  small. 

The  most  that  can  be  said  is  that  plain  shell  boilers  are  de- 
cidedly inferior  to  the  other  forms,  of  which  the  horizontal  return 
tubular  and  the  water-tube  have  ^ven  slightly  higher  results 
than  the  others.  Water-tube  boilers  are  generally  rather  com- 
pacter  and  stand  forcing  better  than  ordinary  tubular  boilers. 
They  also  are  less  likely  to  produce  disastrous  results  if  they 
explode.  On  the  other  hand,  they  are  more  expensive,  and  are 
as  a  class  hard  to  keep  in  good  condition,  particularly  if  the 
water  supply  is  not  of  good  quality. 

Probably  under  average  conditions  a  well-designed  horizon- 
tal return  tubular  boiler  will  give  as  great  evaporative  effi- 
ciency as  can  regularly  be  attained  in  service,  and  the  choice 
between  it  and  a  water-tube  boiler  is  chiefly  in  economy  of 
space  and  capacity  for  forcing.  There  is  no  excuse  for  the 
explosion  of  any  properly  cared  for  boiler. 

The  actual  evaporation  secured  per  poimd  of  total  fuel  is 
something  quite  different  from  the  figures  in  the  table  just 
given.  In  the  first  place,  allowance  must  be  made  for  ash  and 
fuel  used  for  banking  the  fires.  In  the  second  place,  in  regular 
running  the  firing  is  seldom  as  careful  as  in  tests. 

On  account  of  these  the  evaporations  per  poimd  of  com- 
bustible given  in  the  table  must  be  reduced  from  15  to  20  per 
cent  to  correct  the  result  to  pounds  of  coal  used  in  actual 
service. 

Ten  poimds  of  water  or  over,  evaporated  from  and  at  212®  per 
pound  of  total  fuel  may  be  regarded  as  exceptionally  good  prac- 
tice in  every-day  work.  Nine  to  10  lbs.  under  the  same  con- 
ditions represents  fine  average  results,  and  8  to  9  lbs.  is  much 
more  common.  In  fact,  8  lbs.  is  an  unpleasantly  frequent  figure, 
particularly  in  boilers  operated  under  variable  load,  such  as  is 
generally  found  in  electric  plants  of  moderate  size.    All  these 


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ELECTRIC  TRANSMISSION  OF  POWER. 


facts  point  out  the  necessity  of  thorough  and  careful  work  in 
every  part  of  a  power  plant.  Bad  design  or  careless  opera- 
tion anywhere  plays  havoc  with  economy.  In  most  instances 
far  too  little  attention  is  paid  to  the  adaptation  of  the  furnace  to 
the  particular  fuel  used.  In  case  of  attempting  power  trans- 
mission from  cheap  coal  at  or  near  the  mines,  the  furnace  and 
firing  problem  is  of  fimdamental  importance.  Most  furnaces 
are  constructed  to  meet  the  requirements  of  high  grade  fuel 
and  are  quite  likely  to  work  badly  with  anything  else.  In 
transmitting  power  from  cheap  coal  the  grate  surface,  draft, 
and  so  forth  must  be  carefully  arranged  with  reference  to  the 
grade  of  fuel  to  be  used  and  not  with  reference  to  standard 
coals  used  elsewhere.  The  methods  of  firing,  too,  require 
careful  attention. 

At  the  present  time  mechanical  stokers  are  in  very  extensive 
use  in  some  parts  of  the  country.  The  reports  from  them  are 
of  varying  nature,  but  the  consensus  of  opinion  seems  to  be 
that  they  are  very  advantageous  in  working  medium  and  low 
grade  coals,  but  of  less  utility  in  the  case  of  high  grade 
coals.  They  are  somewhat  expensive  and  require  intelligent 
care  now  and  then  like  all  other  machiner}'^,  but  when  it  comes 
to  firing  large  amounts  of  cheap  fuel  at  a  fairly  regular  rate 
they  do  most  excellent  work.  When  coal  is  dear,  careful  hand  * 
firing  is  probably  more  economical  than  any  mechanical 
method.  With  first-class  coal  and  boilers  one  good  fireman 
and  a  coal-passer  can  take  care  of  2,000  KW  in  modem  appara- 
tus, so  that  the  total  cost  of  firing  is  not  a  very  serious  matter. 
A  poor  fireman  is  dear  at  any  price,  and  quite  as  disadvantage- 
ous to  the  station  as  a  poor  engineer. 


Kind  of  Engine. 

Coal  per  IHP  Hour. 

GondenBing. 

Non-Condensing. 

Simple,  dependent  valve, 

2.77 
2.33 
2.22 
2.00 
1.88 
1.66 
1.44 

3.66 

Simple,  independent  valve 

3.11 

Compound,  dependent  valve 

2.66 

Compound,  independent  valve 

2.44 

Ti'inle.  denendent  valve 

Triple,  independent  valve 

Triple,  independent  valve  large 

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ENGINES  AND  BOILERS.  833 

Reverting  now  to  engine  performances,  wo  may  form  a  fairly 
definite  idea  of  what  may  ordinarily  be  expected  in  the  way  of 
coal  consumption  per  indicated  horse-power  hour. 

The  foregoing  table  shows  the  coal  consumption  of  the 
various  kinds  of  engines,  based  on  the  burning  of  1  lb.  of 
coal  for  each  9  lbs.  of  feed  water  used.  Although  greater 
evaporation  can  often  be  obtained,  9  lbs.  of  water  per  pound 
of  coal  is  a  very  good  performance  indeed,  decidedly  better 
than  is  found  in  general  experience.  It  presupposes  good 
boilers,  good  coal,  and  skilful  firing,  such  as  one  has  a  right 
to  expect  in  a  large  power  plant. 

The  figures  apply  only  to  engines  of  several  hundred  HP, 
at  or  near  their  points  of  maximum  economy,  and  operated 
from  a  first-class  boiler  plant. 

They  can  be  and  are  reached  in  regular  working,  and  are 
sometimes  exceeded.  A  combination  of  great  efficiency  at 
the  boilers  and  small  steam  consumption  in  the  engine  some- 
times gives  remarkable  results.  The  best  triple  expansion 
condensing  engines  worked  under  favorable  conditions  can  be 
coimted  on  to  do  a  little  better  than  1.5  lbs.  of  coal  per  IHP 
hour,  occasionally  even  in  the  neighborhood  of  1.25  lbs.  Even 
with  compound  condensing  engines,  tests  are  now  and  then 
recorded,  showing  below  1.5  lbs.  of  coal  per  IHP  hour.  But 
these  very  low  figures  are  the  result  of  the  concurrence  of 
divers  very  favorable  conditions,  and  those  just  tabulated 
are  as  good  as  one  should  ordinarily  expect.  It  must  not  be 
supposed  that  the  weight  of  coal  used  per  HP  hour  necessarily 
determines  the  economy  of  the  plant.  The  cost  of  fuel  of 
course  varies  greatly,  and  its  price  in  the  market  is  by  no 
means  proportional  to  its  thermal  value.  As  a  rule,  the  coals 
which  give  the  best  economic  results  are  not  those  of  the 
greatest  intrinsic  heating  power.  On  the  contrary,  dollar  for 
dollar,  the  best  results  are  veyy  frequently  obtained  from  cheap 
coal,  or  mixtures  of  inferior  coal  with  a  portion  of  a  better 
grade.  Hence,  the  boilers  of  a  plant  which  is  a  model  of  econ- 
omy may  show  an  evaporation  of  only  7  or  8  lbs.  of  water 
per  pound  of  coal.  Boiler  tests  with  the  conditions  of  economy 
in  view  are  of  great  importance,  and  are  likely  to  pay  for  them- 
selves tenfold  in  even  a  few  months  of  operation. 


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334  ELECTRIC  TRANSMISSION  OP  POWER. 

A  word  here  about  fuel  oil.  Petroleum  has,  weight  for 
weight,  much  greater  heating  power  than  coal.  Its  heat  of 
combustion  is  about  20,000  to  21,000  thermal  units,  it  costs 
little  to  handle  and  fire,  leaves  no  ash  and  refuse  to  be  taken 
care  of,  produces  little  smoke,  and  is  generally  cleanly  and 
convenient. 

It  has  been  thoroughly  tried  by  some  of  the  largest  elec- 
trical companies  in  this  coimtry,  and  at  moderate  prices,  a 
dollar  a  barrel  or  less,  is  capable  of  competing  on  fairly  even 
terms  with  coal.  But  experience  has  shown  some  curious 
facts  about  its  performance.  The  amount  of  steam  or  equiva- 
lent power  required  to  inject  and  vaporize  the  oil  in  one  of 
the  most  skilfully  handled  plants  in  existence  amoimts  to  no 
less  than  7i  per  cent  of  the  total  steam  produced.  And 
curiously  enough,  the  cost  of  oil  for  firing  up  a  fresh  boiler, 
and  the  time  consumed,  compare  unfavorably  T^ith  the  results 
obtained  from  coal.  In  spite  of  the  great  amount  of  heat 
evolved  from  fuel  oil,  it  appears  to  be  less  effective  than  coal 
in  giving  up  this  heat  to  the  boiler  by  radiation  and  convection. 
There  is  good  reason  to  believe  that  more  than  half  the  total 
heat  of  combustion  of  incandescent  fuel  is  given  off  as  radiant 
heat,  and  most  of  the  remainder  is  of  course  transferred  by 
convection  both  of  heated  particles  of  carbon  and  of  molecules 
of  gas. 

It  is  not  imlikely,  therefore,  that  a  petroleum  fire  with  its 
small  radiating  power  and  comparative  absence  of  incan- 
descent particles,  fails  in  economy  through  inabiUty  to  give 
up  its  heat  readily.  This  view  of  the  case  is  bonie  out  by  the 
facts  above  cited  and  by  the  abnormally  high  temperature  of 
the  escaping  gases  often  foimd  in  boiler  tests  with  petroleum 
fuel.  At  all  events  it  is  clear  from  such  tests  that  the  evapo- 
ration obtained  from  fuel  oil  is  not  so  great  as  would  be  ex- 
pected from  its  immense  heat  of  combustion,  and  unless  at 
an  exceptionally  low  price,  its  use  is  less  economical  than  that 
of  coal. 

The  most  striking  innovation  of  recent  years  in  the  genera- 
tion of  mechanical  power  by  steam  is  the  development  of  the 
steam  turbine.  Year  by  year  during  the  past  decade  it  has 
slowly  grown  from  experiment  to  realization,  until  at  the  pres- 


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ENGINES  AND  BOILERS.  335 

ent  time  it  has  reached  a  position  that  demands  for  it  most 
serious  consideration.  It  looks  very  much  as  if,  for  many  pur- 
poses, the  reciprocating  steam  engine  might  be  hard  pushed. 

Strangely  enough  the  steam  turbine  or  impulse  wheel  is  the 
earliest  recorded  form  of  steam  engine,  dating  clear  back  to 
Hero  of  Alexandria,  who  flourished  about  130  B.C.  The  engine 
which  Hero  suggested  was  merely  a  philosophical  toy,  and  it 
took  nineteen  centuries  beyond  his  day  to  produce  any  engine 
that  was  not  a  toy,  but  now  after  two  thousand  years  Hero's 
idea  has  borne  fruit. 


r''- 


T^Tt 


Fio.  191. 

The  fundamental  principle  of  the  steam  turbine  is  just  that 
of  the  water  turbine  —  directing  fluid  pressure  against  a  series 
of  rotating  buckets.  The  first  practical  steam  turbine,  devised 
by  De  Laval,  is  very  closely  akin  to  the  Pelton  water-wheel 
and  to  the  little  water-motors  sometimes  attached  to  faucets 
for  furnishing  a  small  amount  of  power.  The  essential  fea- 
tures of  his  apparatus  are  well  shown  in  Fig.  191.  It  consists 
of  a  narrow  wheel  A  with  buckets  around  its  periphery,  re- 
volving within  a  housing  B  and  supported  by  a  rather  long  and 
slender  shaft.  Bearing  upon  the  buckets  at  an  acute  lateral 
angle  is  the  steam  jet  Ey  in  this  case  one  of  three  equidistant 
jets  playing  on  the  same  wheel.    To  obtain  the  most  efficient 


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836  ELECTRIC  TRANSMISSION  OF  POWER, 

working  of  the  jet  the  steam  nozzle  is  somewhat  contracted  at 
D,  a  little  way  back  from  the  buckets.  The  steam  is  discharged 
on  the  other  side  of  the  wheel  as  shown.  It  strikes  the  buckets 
as  a  jet  at  great  velocity,  and  should,  if  the  conditions  were 
just  right,  expend  nearly  all  its  energy  in  driving  the  wheel  and 
should  itself  leave  it  at  or  near  zero  velocity.  Of  course  thio 
condition  does  not  hold  in  practice,  but  still  a  steam  turbine  of 
this  De  Laval  construction  is  capable  of  doing  marvellously  well. 
The  main  objection  to  this  form  is  the  enormously  high  rotative 
speeds  necessary  for  efficient  running.  Here,  as  in  hydraulic 
impulse  wheels,  the  peripheral  velocity  should  be  about  one-half 
the  spouting  velocity  of  the  fluid.  With  high  pressure  steam 
this  is,  when  one  works  the  turbine  condensing,  3,000  to  5,000 
feet  per  second.  In  practice  these  De  Laval  wheels  have  usu- 
ally been  geared  to  a  driving  shaft,  but  the  wheel  itself  has  run 
at  10,000  to  30,000  r.p.m.,  seldom  below  the  former  figure  even 
in  large  sizes.  But  the  economy  reached  has  sometimes  been 
very  high,  as  in  some  tests  a  few  years  ago  in  France,  when, 
with  an  initial  pressure  of  192  lbs.,  a  300  HP  turbine  showed 
a  steam  consumption  of  only  13.92  lbs.  per  effective  HPH  — 
a  figure  seldom  reached  with  engines.  The  governing  is  by 
throttling  the  steam  supply  in  response  to  the  movement  of 
a  fly-ball  governor  of  the  kind  generally  familiar  in  steam 
engineering. 

The  inconvenience  of  the  very  high  rotative  speed  of  such 
turbines  has  led  to  the  development  of  forms  working  more 
along  the  lines  of  hydraulic  turbines,  of  which  by  far  the  best 
known  is  the  Parsons  turbine,  which  has  recently  made  so 
striking  a  record  in  marine  work,  having  been  applied  to  sev- 
eral British  torpedo-boats  and  even  to  larger  vessels.  In  this 
remarkable  machine  the  passage  of  the  steam  is  parallel  to 
the  axis  of  rotation  instead  of  tangential,  and  its  hydraulic 
prototype  is  the  parallel-flow  pressure  turbine,  shown  in  dia- 
gram in  Fig.  200.  In  fact  the  steam  is  passed  successively 
through  a  large  number  of  such  parallel-flow  turbines,  gradually 
expanding  and  giving  up  its  energy  to  the  successive  runners 
located,  of  course,  on  the  same  shaft.  The  course  of  the  ex- 
panding steam  is  well  shown  in  Fig.  192,  which  gives  in  diagram 
its  progression  through  four  sets  of  vanes,  two,  1  and  3,  being 


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ENGINES  AND  BOILERS. 


837 


rings  of  guide  blades,  and  the  others,  2  and  4,  rings  of  ninner 
blades.  The  steam,  starting  at  pressure  P,  expands  succes- 
sively to  Pi,  Pii,  Piii,  Piv,  expanding  sharply  against  the  runner 
blades  and  giving  them  a  reactive  kick  as  it  leaves.  In  this 
case  the  steam  velocity  against,  say  the  runner  blades  2,  is 
not,  as  in  the  De  Laval  form,  the  full  spouting  velocity  due 
to  the  initial  head  of  steam,  but  merely  that  corresponding  to 
the  differential  pressure  P-Pi.  This  enables  the  peripheral 
speed  of  the  rimner  to  be  kept  within  reasonable  limits  without 
violating  the  conditions  of  economy,  but  the  turbine  at  best 
is  not  a  slow-speed  machine. 

Fig.    193   is   a   longitudinal   section   through   the    Parsons 
type  of  steam  turbine  as  developed  in  this  country  by  the 


station) 


cccccc^cc 


FT 


ccccccc^cc 


StationiuT^ 


J)  i) ))  j) ))  3)  i) ))  > 

Fig.  192. 


XovlngBlAdM 


Moving  Blades 


Westinghouse  Machine  Co.  The  steam  enters  from  the  supply 
pipe  controlled  by  the  governor  and  comes  first  into  the  annu- 
lar chamber  A  at  the  extreme  left-hand  end  of  the  runner. 
The  runner  blades  are  graduated  in  size  so  that  the  expanding 
steam  may  give  nearly  a  uniform  useful  pressure  per  blade, 
and  to  this  end  the  diameter  of  the  runner  hub  is  twice  in- 
creased as  the  steam  expands  towards  the  exhaust  chamber  B. 
The  endwise  thrust  of  the  steam  entering  the  turbine  from  A 
is  balanced  by  its  equal  pressure  on  the  balancing  piston  C, 
which  revolves  with  the  runner.  To  the  left  of  this  is  an 
annular  steam  space  and  a  second  balance  piston  C.  Now, 
when  the  expanding  steam  has  passed  the  first  section  of  the 
runner  into  the  steam  space  E,  it  can  flow  back  through  the 
channel  F  and  the  post  D  against  this  second  balance  piston. 


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ass 


ELECTRIC  TRANSMISSION  OP  POWER, 


e  It'  > 


Still  further  to  the  left  is  a  second  steam  space  and  a  third 
piston  C,  which  is  similarly  exposed  to  the  pressure  from  G, 
where  the  third  stage  of  expansion  begins.    The  effect  of  this 


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ENGINES  AND  BOILERS,  889 

balancing  system  is  to  render  the  end  thrust  negligible  what- 
ever may  be  the  ratio  of  expansion  in  the  turbine.  A  thrust 
bearing  at  H  keeps  the  working  parts  positioned  and  takes  up 
the  trivial  thrusts  which  may  incidentally  be  present.  J,  J,  J 
are  the  bearings,  which  are  out  of  the  ordinary  in  that  within 
the  gun-metal  sleeve  that  forms  the  bearing  proper  are  three 
concentric  sleeves  fitting  loosely.  The  clearance  between 
them  fills  with  oil,  cushioning  the  bearings.  Now  if  the  run- 
ner is  not  absolutely  in  balance  there  is  a  certain  flexibility  in 
the  bearings  so  that  the  runner  can  rotate  about  its  centre 
of  gravity  instead  of  its  geometrical  centre,  thus  stopping 
vibration.  An  equivalent  expedient  is  found  in  the  De  Laval 
turbine,  the  shaft  of  which  is  deliberately  made  slightly  flexible 
so  that  it  may  take  up  rotation  about  its  centre  of  gravity. 
If  is  a  pipe  which  again  takes  up  the  work  of  keeping  the 
pressure  balanced  by  connecting  the  exhaust  chamber  B  \nth 
the  steam  space  behind  the  last  balance  piston.  At  M  is  a 
simple  oil  pump  taking  oil  from  the  drip  tank  N  and  lifting  it 
to  the  tank  0,  whence  it  is  distributed  to  the  bearings.  A  by 
pass  valve  P  turns  high  pressure  steam  directly  into  the  steam 
space  E,  in  case  a  very  heavy  load  must  be  carried,  or  a  con- 
densing turbine  temporarily  rim  non-condensing,  i?  is  a 
flexible  coupling  for  the  driving  shaft,  and  at  that  point  too  is 
the  worm  gear  that  drives  the  governor.  The  governor  in 
its  operation  is  somewhat  peculiar.  Instead  of  throttling  the 
steam  supply  so  as  to  reduce  the  effective  pressure,  the  steam 
is  always  sent  to  the  turbine  at  full  boiler  pressure,  but  discon- 
tinuously.  The  main  steam  valve  is  controlled  by  a  little 
steam  relay  valve  which  is  given  a  regular  oscillatory  motion 
by  a  lever  driven  from  an  eccentric.  The  steam  is  thus  ad- 
mitted to  the  turbine  in  a  series  of  periodic  puffs.  Now  the 
fulcrum  of  this  valve  lever  is  movable  and  is  positioned  by 
the  fly-ball  governor,  so  that  the  position  of  the  valve  with  rela- 
tion to  the  port  is  varied  without  changing  the  rate  or  ampli- 
tude of  the  valve  motion.  Hence  the  length  of  the  puffs  is 
changed  so  that  while  at  full  load  steam  is  on  during  most  of 
the  period,  at  light  load  it  is  on  for  only  a  small  part  of  each 
period.  This  is  well  shown  graphically  by  Fig.  194,  which  is 
self-explanatory. 


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340 


ELECTRIC  TRANSMISSION  OF  POWER. 


The  governor  balls  are  so  arrajiged  that  they  work  both 
ways,  their  mid-position  corresponding  to  full  admission  of 
steam,  so  that  a  violent  overload  can  be  made  to  shut  off  steam, 
and  a  break  in  the  governor  driving  gear  will  do  the  same 
instead  of  letting  the  turbine  run  away. 

These  turbines  are  capable  of  operating  with  really  remark- 
able efficiency.  Fig.  195,  from  the  makers'  tests,  gives  the 
performance,  both  condensing  and  non-condensing,  of  a  West- 
inghouse-Parsons  turbine  directly  coupled  to  a  300  KW 
quarter-phase  alternator  giving  440  volts  at  60*^,  the  speed  be- 
ing 3,600  r.p.m.     Operated  condensing,  the  steam  consumption 


WHEN  RUNNING  LIGHT  LOAD 

Fio.  194. 

at  full  load  falls  to  about  16.4  lbs.  per  electrical  HPH,  and  is 
below  20  lbs.  from  125  HP  up.  This  extraordinary  uniformity 
of  performance  at  large  and  small  loads  is  mainly  due  to  the 
very  small  frictional  losses  in  the  turbine,  although  it  is  helped, 
perhaps,  by  the  load  curve  of  the  generator.  The  results  when 
working  non-condensing  are  very  much  inferior  to  these,  rela- 
tively worse  than  in  an  ordinary  steam  engine. 

Altogether  it  is  an  admirable  showing  for  the  steam  turbine. 
The  writer  believes  Fig.  195  to  be  entirely  trustworthy,  as  it 
corresponds  very  closely  ^vith  certain  independent  tests  now 
in  his  possession,  from  another  turbine  of  the  same  capacity 
and  speed,  in  which  tests  the  makers  of  the  turbine  had  no 


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PLATE  XI. 


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ENGINES  AND  BOILERS, 


841 


part.  The  substance  of  the  matter  is  that  the  steam  turbine 
will  work  just  about  as  efficiently  as  a  first-class  compound 
condensing  engine,  and  can  not  only  be  more  cheaply  made, 
but  takes  up  much  less  room.  In  the  same  way,  for  electri- 
cal, purposes  a  directly  connected  generating  set  with  steam 
turbine  is,  or  ought  to  be,  much  cheaper  and  smaller  than 
those  now  in  use,  while  retaining  equally  high  efficiency. 
High  rotative  speed  is  by  far  the  cheapest  way  of  getting  out- 


as   so 


75    100126150    175    200225290275800885850875400426 

ELECTRJOAL  HOME  POWER 

FlO.  UB.^ 


put,  and  when,  as  in  this  case,  no  heavy  reciprocating  parts 
are  involved,  there  is  no  good  reason  for  objecting  to  high 
speed.  The  present  fashion  for  low  speed  dynamos  is  largely 
a  fad,  having  its  origin  in  direct  coupling  to  Corliss  engines, 
and  with  the  modem  stationary  armature  construction  there  is 
no  reason  why  high  rotative  speed  should  not  be  used,  at  least 
in  alternators. 

In  Plat«  XI  is  shown  the  first  large  turbine-driven  generator 
installed  for  regular  commercial  service  in  this  country.  Smaller 
ones  had  been  in  use  in  isolated  plants  for  some  time,  but 


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342  ELECTRIC  TRANSMISSION  OF  POWER, 

this  1,500  KW  set.  installed  for  the  Hartford  Electric  Light 
Co.,  was  the  first  important  installation  of  this  kind.  The 
turbine  is  designe<l  for  a  maximum  output  of  3,000  HP  at  a 
speed  of  1,200  r.p.m.,  and  the  complete  set,  weighing  only 
175,000  lbs.,  takes  a  floor  space  of  but  33'  3^^  X  8'  9^.  The 
generator  is  a  quarter-phase  machine  at  60^^  frequency.  This 
outfit  should  be  capable  of  giving  an  efficiency  rather  better 
than  that  shown  in  Fig.  195  —  probably  less  than  15  lbs.  of 
steam  per  electrical  HPH  at  steady  full  load;  in  other  words,  it 
should  do  nearly  as  well  as  a  triple  expansion  engine.  This 
machine  has  now  been  in  successful  operation  for  some  four 
years. 

Alternators  may  be  conveniently  and  cheaply  built  for  the 
speed  implied  in  steam  turbine  practice,  but  continuous  cur- 
rent generators  involve  some  difficulties.  For  power  trans- 
mission work  turbo-generators  have  much  to  recommend  them 
as  auxiliaries,  and  there  is  a  strong  probability  of  their  taking  an 
important  place  in  the  development  of  the  art.  There  has  not 
yet  been  accumulated  enough  experience  with  them  to  enable 
a  final  judgment  of  their  practical  properties  to  be  formed, 
or  to  justify  an  unqualified  indorsement  of  their  economy. 

Another  successful  form  of  steam  turbine,  now  consider- 
ably used  in  units  of  output  as  great  as  several  thousand  KW, 
is  the  Curtis,  which  differs  radically  in  several  respects  from 
that  just  described.  In  the  first  place  it  is  not  a  pressure 
turbine  but  an  impulse  turbine,  in  which  the  steam  is  expanded 
in  the  admission  nozzles  to  a  high  jet  velocity,  the  kinetic 
energy  of  which  is  then  utilized  in  the  runner  buckets.  It 
is  thus  dynamically  more  akin  to  the  De  Laval  than  to  the 
Parsons  turbine,  but  differs  from  it  much  as  a  true  impulse 
turbine  differs  from  a  Pel  ton  wheel. 

In  the  second  place  it  is  regularly  built  in  all  the  larger 
sizes  as  a  vertical  shaft  machine  carrying  the  generator  arma- 
ture on  its  upper  end  and  being  supported  below  by  an  inge- 
niously contrived  water  step  kept  afloat  by  a  pressure  pump. 
In  the  Curtis  turbine,  however,  the  expansion  takes  place 
not  in  a  single  nozzle  as  in  the  De  Laval  type  but  in  several 
success' ve  stages  so  that  the  jet  velocity  and  with  it  the  neces- 
sary peripheral  speed  is  very  considerably  reduced.     Fig.  196 


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ENGINES  AND  BOILERS, 


843 


shows  the  course  of  the  steam  and  is  almost  self-explanatory. 
The  upper  section  is  the  first  stage,  the  lower  the  second  stage, 
and  in  some  of  the  larger  units  three  or  four  stages  are  used. 
Their  effect  is  akin  to  that  of  compounding  an  engine  in  the 
better  temperature  distribution  and  proportioning  of  parts. 
The  governing  in  this  turbine  is  exactly  in  line  with  the  prin- 


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Fio.  196. 

ciples  of  hydraulic  impulse  turbines;  the  numerous  admission 
nozzles  being  fitted  with  independent  valves  as  sho\vn  in  the 
cut.  These  are  in  succession  opened  or  closed  in  accordance 
with  the  requirements  of  the  load,  by  the  action  of  a  sensitive 
fly-ball  governor  which  controls  a  series  of  relay  valves,  in  turn 
working  the  admission  valves.     In  the  earlier  and  some  of 


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344  ELECTRIC  TRANSMISSION  OF  POWER. 

the  later  turbines  these  relay  valves  have  been  electrically 
actuated,  but  at  present  both  this  and  purely  mechanical  con- 
trol are  used.  As  each  admission  valve  is  either  fully  open  or 
closed  there  is  no  throttling  of  the  steam,  which  gives  a  material 
gain  in  efficiency. 

Plate  XII  shows  a  500  KW  Curtis  turbo-generator  which 
is  peculiarly  interesting  as  being  a  direct  current  machine 
designed  for  railway  purposes.  The  rotative  speed  is  1,800 
r.p.m.,  but  in  spite  of  this,  the  problem  of  commutation  has 
been  successfully  met.  In  this,  as  in  all  the  large  Curtis  turbines, 
there  is  but  one  supporting  bearing  on  which  the  moving  parts 
spin  top  fashion  kept  in  line  by  a  pair  of  small  guide  bearings. 
The  structure  is  thus  wonderfully  compact,  but  it  is  still  an 
open  question  among  engineers  as  to  the  advisability  of  a 
vertical  shaft.  It  gains  a  little  in  friction  and  a  certain  amount 
in  space  together  with  immunity  from  flexure  of  shaft,  while 
on  the  other  hand  it  loses  greatly  in  accessibility,  and  exposes 
the  generator  portion  to  certain  risks  from  heat,  steam,  and  oil 
that  are  not  altogether  negligible.  From  a  practical  stand- 
point the  Curtis  turbine  has  made  a  good  record,  and  many 
large  units,  even  up  to  5,000  KW,  are  in  successful  use,  to  no 
small  extent  in  large  stations  designed  for  railway  service, 
polyphase  current  being  generated  and  transmitted  to  con- 
verter stations.  The  large  polyphase  turbo-generators  are  all 
of  the  vertical  type  closely  resembling  Plate  XII. 

The  strong  points  of  steam  turbines  are  cheapness  for  a 
given  output,  economy  of  floor  space,  freedom  from  vibration, 
uniform  efficiency  at  various  loads,  and  light  friction.  Of 
these  the  first  is  the  direct  result  of  high  speed  both  in  tiu"bine 
and  generator.  For  some  years  there  was  a  strong  tendency 
toward  very  low  engine  speeds,  which  produced  generating 
units  of  needlessly  great  weight  and  cost.  The  turbine  goes 
to  the  other  extreme  of  speed,  and  while  it  runs  at  speed  too 
high  for  the  most  economical  construction,  can  be  built  in- 
cluding the  generator  at  a  cost  probably  below  that  of  any 
other  direct  connected  unit.  The  current  price  is  relatively 
high,  being  adjusted  to  that  determined  by  engines,  but  this 
condition  is  of  course  temporary.  Economy  of  floor  space  is 
very  marked,  especially  in   the  vertical    shaft   type,   but    it 


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PLATE   XII. 


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ENGINES  AND  BOILERS,  845 

actually  is  much  less  than  at  first  appears,  since  the  location 
of  the  boilers  generally  determines  the  area  of  the  plant,  and 
turbines  cannot  conveniently  be  huddled  into  the  space  which 
their  dimensions  would  suggest.  They  are  remarkably  free 
from  vibration,  due  to  the  necessity  of  avoiding  centrifugal 
strains  by  extremely  careful  balancing,  and  their  friction  is 
very  light  indeed.  The  uniform  economy  at  various  loads  is 
partly  due  to  small  friction  and  partly  to  the  fact  that  the 
expansion  of  the  steam  is  substantially  fixed  by  the  construc- 
tion and  does  not  vary  materially  with  the  load.  Nevertheless, 
as  has  already  been  shown,  an  engine  properly  designed  for 
varying  load  will  show  at  least  equally  good  practical  results. 
Compare  in  this  the  lowest  curve  of  Fig.  186,  and  Fig.  195,  to 
say  nothing  of  Fig.  187. 

The  actual  efficiency  of  the  steam  turbine  is  good  without 
being  in  any  way  phenomenal.  At  equal  steam  pressure,  super- 
heat, and  vacuum,  the  steam  turbine,  in  its  present  stage  of 
development,  is  as  a  rule  slightly  less  efficient  per  brake  horse- 
power than  a  first-class  compound  condensing  engine.  Tur- 
bines, however,  suffer  extremely,  like  other  engines  in  which 
there  is  very  great  expansion,  from  diminished  vacumn  and 
only  by  using  a  vacuum  of  28^^  and  100°  to  150°  F.  superheat- 
ing, can  they  be  brought  up  to  a  performance  just  about  equiva- 
lent to  a  compound  condensing  engine.  No  tests  of  turbines 
made  under  any  conditions  have  yet  been  able  to  equal  or  very 
closely  approach  the  best  results  from  compound  or  triple  ex- 
pansion reciprocating  engine  in  steam  per  brake  horse-power. 
Whether  they  will  do  so  in  the  future  remains  to  be  seen,  but 
at  present  there  is  no  reason  to  predict  it. 

As  a  practical  matter,  however,  one  can  very  comfortably 
stand  some  loss  in  efficiency  if  thereby  the  fixed  charges  and 
maintenance  can  be  kept  down.  The  greater  the  necessary 
proportion  of  such  items,  the  more  complacently  can  one 
stand  a  slight  loss  in  steam  economy.  In  the  case  of  engines 
which  from  the  conditions  of  their  use  give  a  rather  small 
annual  output  for  their  capacity,  reduction  of  fixed  charges 
and  maintenance  is  of  great  importance.  Hence  for  auxiliary 
plants  in  power  transmission  which  are  idle  a  large  part  of  the 
time,  there  is  a  great  deal  to  be  said  for  the  turbo-generator  if 


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346 


ELECTRIC  TRANSMISSION  OF  POWER. 


it  can  be  had  at  a  reasonable  figure.  It  can  also  be  put  into 
action  more  quickly  than  ordinary  engines,  and  handles  heavy 
overloads  well  beside  running  easily  in  parallel  by  reason  of 
the  uniform  rotative  effort. 

Considerable  space  has  here  been  given  to  describing  some 
of  the  details  of  steam  turbines,  in  the  belief  that  they  are  of 


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sufficient  importance  to  warrant  it  even  in  a  chapter  not 
intended  to  be  in  the  least  a  compendium  of  steam  practice, 
but  a  mere  outline  of  the  essential  facts.  They  certainly  have 
already  proved  their  right  to  a  place,  and  the  question  is  now 
merely  that  of  the  probable  limitations  of  their  usefulness, 
which  only  protracted  experience  can  disclose. 

For  an  electrical  power  station  operated  by  steam  power  the 
vital  economical  question  is  the  cost  of  fuel  per  kilowatt  hour. 


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ENGINES  AND  BOILERS. 


847 


rather  than  the  performance  of  engines  and  boilers  alone. 
This  final  result  involves  the  performance  of  the  station  appa- 
ratus imder  varying  loads,  too  frequently  rather  light,  and, 
implicitly,  the  skill  of  the  operator  in  keeping  his  apparatus 
actually  running  as  near  its  point  of  maximum  economy  as 
possible,  in  spite  of  changes  in  the  electrical  output.  This 
personal  element  forbids  a  reduction  of  the  facts  to  general 
laws,  but  a  concrete  example  will  be  of  service  in  showing  what 
may  be  expected  in  a  well-designed  and  well-operated  power 
plant.  Fig.  197  shows  a  pair  of  *'  load  lines,"  from  a  large  and 
particularly  well-operated  power  plant.     The  solid  line  shows 


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Fxo.  198. 


the  variations  of  load  throughout  a  day  in  the  latter  end  of 
January,  and  the  broken  line  the  variations  of  load  during  a 
day  early  in  April. 

The  early  darkness  of  a  winter's  day  is  very  obvious  in  the 
former  line.  The  station  carries  in  addition  to  lights  a  heavy 
motor  service  that  keeps  up  a  fairly  miiform  load  through- 
out the  day,  until  the  sudden  call  for  lights  in  the  early 
evening.  The  load  factor  shown  by  the  solid  line  is  .35  (i.e., 
this  is  the  ratio  between  maximum  and  average  load).  The 
second  load  line  gives  a  much  better  relation  between  these 
quantities,  the  load  factor  being  .64,  which  is  quite  usual  in 
this  station  during  the  spring  and  summer.  Of  course  every 
effort  is  exerted  to  keep  the  machines  which  are  in  use  as  fully 


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848  ELECTRIC  TRANSMISSION  OF  POWER. 

loaded  as  possible.  In  spite  of  this  the  small  output  during 
the  early  morning  hours,  coupled  with  the  losses  due  to  circu- 
lating pumps  and  other  minor  machinery,  and  the  fuel  used  for 
banking  and  starting  fires,  brings  the  cost  of  fuel  during  this 
period  far  above  the  average  for  the  day.  The  curve  in  Fig. 
198  shows  roughly  the  variation  in  the  cost  of  fuel  per  KW- 
hour  throughout  the  day,  taken  from  the  average  of  a  nimi- 
ber  of  tests.  As  the  fuel  cost  in  a  large  central  station  is  a 
considerable  portion  of  the  total  expense,  it  is  evident  that  the 
result  is  an  excellent  one.  During  all  the  hours  of  heavy  load 
the  cost  of  fuel  is  less  than  six-tenths  of  a  cent  per  KW-hour, 
and  the  total  cost  of  production  but  little  more.  This  result 
will  give  an  excellent  idea  of  the  cost  of  generating  power  on 
a  large  scale  with  cheap  coal.  It  is,  however,  exceptionally 
good,  and  can  only  be  equalled  by  a  very  well  managed  plant 
with  the  best  modem  equipment  both  electrical  and  mechanical. 
Of  course  the  expenses  of  distribution,  administration,  and 
the  like  must  be  taken  into  account  in  considering  the  cost 
per  KW  hour  delivered.  The  general  question  of  station  ex- 
penses cannot  be  here  investigated,  but  this  brief  digression 
gives  some  idea  of  the  necessary  relation  between  the  character 
of  the  work  and  the  commercial  results  in  generating  electric 
power  on  a  large  scale,  so  far  as  the  use  of  steam  engines  as 
prime  movers  is  concerned. 


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CHAPTER  IX. 

WATER-WHEELS. 

The  importance  of  the  development  of  water-powers  for 
electrical  purposes  we  have  already  come  fully  to  realize.  The 
lessons  of  the  last  few  years  have  been  exceedingly  valuable 
ones,  and  it  is  safe  to  say  that  the  utilization  of  water-powers 
for  electrical  transmission  will  be  kept  up  until  every  one 
which  is  capable  of  commercially  successful  development  is 
worked  to  its  utmost  capacity.  In  spite  of  the  length  of  time 
that  water-wheels  of  various  sorts  have  been  used,  it  is  only 
very  recently  that  these  prime  movers  have  been  brought  to  a 
stage  of  development  that  renders  them  satisfactory  for  elec- 
trical purposes.  The  old  water-wheel  was  even  more  trouble- 
some as  a  source  of  electrical  power  than  the  old  slide  valve 
steam  engine. 

The  customary  classification  of  water-wheels  for  many  years 
has  been  into  overshot,  undershot,  and  breast-wheels,  and 
finally  tiu-bines.  Various  modifications  of  all  these  have,  of 
course,  been  proposed  and  used.  Of  these  classes,  the  first 
three  may  be  passed  over  completely  as  having  no  importance 
whatever  in  electrical  matters,  save  in  certain  modifications  so 
different  from  the  original  wheel  as  to  be  scarcely  recognizable. 
To  all  intents  and  purposes  they  are  never  used  for  the  pur- 
pose of  driving  dynamos,  although  occasionally  an  isolated 
instance  appears  on  a  very  small  scale. 

It  is  the  turbine  water-wheel  which  has  made  modem 
hydraulic  developments  possible,  and  more  particularly  elec- 
trical developments.  The  turbine  practically  dates  from  1827, 
when  Foumeyron  installed  the  first  examples  in  France, 
although  it  is  interesting  to  know  that  a  United  States  patent 
of  1804  shows  a  wheel  of  somewhat  similar  description,  never 
so  far  as  is  known  used.  The  modem  turbine  consists  of  two 
distinct  parts,  the  system  of  guide  blades  and  the  runner.  The 
runner  is  the  working  part  of  the  wheel,  and  consists  of  a 

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series  of  curved  buckets  so  shaped  as  to  receive  the  water  with 
as  little  shock  as  practicable,  and  to  reject  it  only  after  having 
utilized  substantially  all  of  its  energy.  These  buckets  are 
arranged  in  almost  every  imaginable  way  around  the  axis  of 
the  runner,  but  always  symmetrically. 

Sometimes  the  curvature  of  the  buckets  is  such  that  the 
water  after  having  passed  through  them  leaves  the  wheel 
parallel  to  its  axis;  sometimes  so  that  the  water  flows  inward 
and  is  discharged  at  the  centre  of  the  nmner;  sometimes  so 
that  it  passes  outward  and  is  discharged  at  the  periphery. 


FlO.  199. 


Fio.  200. 


More  often  the  buckets  have  a  double  curvature  so  that  the 
water  flows  along  the  axis  and  at  the  same  time  either  inwardly 
or  outwardly.  It  is  not  unusual,  moreover,  to  have  two  sets  of 
buckets  on  the  same  shaft  for  various  purposes.  The  growth 
of  the  art  of  turbine  building  has  made  any  classification  of 
turbines  depending  on  the  direction  of  the  flow  of  the  water 
very  uncertain,  as  in  nearly  every  American  turbine  this  flow 
takes  place  in  more  than  one  general  direction,  usually  inward 
and  downward.  Aside  from  the  runner  the  essential  feature 
of  the  modem  turbine  is  the  set  of  guide  blades  which  sur- 
round the  runner,  and  which  are  so  curved  as  to  deliver  the 
water  fairly  to  the  buckets  in  such  direction  as  will  enable  it 
to  do  the  most  good.  Accordingly  these  blades  are  ciu-ved  in 
all  sorts  of  ways,  according  to  the  way  in  which  the  water  is 
intended  to  be  utilized. 

Fig.  199,  taken  from  Rankine,  shows  a  species  of  idealized 
turbine  which  discloses  the  principles  very  clearly.  In  this  fig- 
ure A  is  the  guide  blade  system  and  B  the  runner.-  The  flow 
is  entirely  along  the  axis,  forming  the  so-called  parallel  flow 
turbine,  a  form  not  in  general  use  in  America. 


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WATER-WHEELS.  861 

Fig.  200  shows  the  sort  of  curvature  which  is  given  to  the 
guide  blades  and  to  the  buckets  of  the  runner.  The  axis  of 
this  or  any  other  kind  of  turbine  may  be  horizontal  or  vertical, 
as  convenience  dictates.  As  may  be  judged  from  the  illustra- 
tion, the  water  acts  on  the  runner  with  a  steady  pressure,  and 
the  buckets  of  the  runner  are  always  filled  with  the  water 
which  drives  them  forward.  Working  in  this  way  by  water 
pressure  due  to  the  weight  of  the  water  column,  it  is  not 
necessary  that  the  turbine  should  be  placed  at  the  extreme 
bottom  of  the  fall,  provided  an  air-tight  casing  is  continued 
below  the  runner  so  as  to  take  advantage  of  the  solid  water 
column  below  the  turbine.  Such  an  arrangement  is  called  a 
draft  tube,  and  may  be  of  any  length  up  to  the  full  column 
which  may  be  supported  by  atmospheric  pressure,  provided 
the  body  of  water  shall  be  continuous  so  that  there  shall  be  no 
loss  of  head  due  to  the  drop  of  the  water  from  the  wheel  to  the 
level  of  the  water  in  the  tail-race.  It  is  as  if  the  column  below 
were  pulling  and  the  column  above  pushing,  the  ruimer  being 
in  a  solid  stream  extending  from  the  highest  to  the  lowest 
level  of  water  used.  As  a  matter  of  practice  the  draft  tube  is 
generaUy  made  considerably  shorter  than  the  column  of  water 
which  might  be  supported  by  atmospheric  pressure,  generally 
less  than  20  feet,  depending  somewhat  on  the  size  of  the  wheel. 
With  longer  tubes  it  is  difficult  to  preserve  a  continuous  column, 
which  is  necessary  in  order  to  utilize  the  full  power  of  the 
water. 

Nearly  all  American  turbines  are  of  this  so-called  "pressure" 
type.  There  is,  however,  another  type  of  turbine  wheel  used 
somewhat  extensively  abroad,  and  occasionally  manufactured 
in  this  country,  which  without  any  very  great  change  in 
character  of  the  structure  operates  on  an  entirely  different 
principle.  There  are  present,  as  before,  guide  blades  deliver- 
ing the  water  to  the  buckets  of  the  ruimer,  but  the  spaces  be- 
tween these  blades  are  so  shaped  and  contracted  as  to  deliver 
the  water  to  the  runner  as  a  powerful  jet.  The  energy  of 
water  pressure  is  converted  into  the  kinetic  energy  of  the 
spouting  jet,  and  the  buckets  of  the  nmner  are  not  filled  solidly 
and  smoothly  with  the  water,  but  serve  to  absorb  the  kinetic 
energy  of  the  jets,  and  discharge  the  water  below  at  a  very 


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ELECTRIC  TRANSMISSION  OF  POWER. 


low  velocity.  Such  turbines  are  known  as  impulse  turbines, 
from  the  character  of  their  action.  In  the  pressure  turbines  the 
full  water  pressure  acts  in  the  runner  and  in  the  space  between 
the  guides  and  the  runner.  In  the  pressure  turbine  each  space 
between  the  guide  blades  acts  so  as  to  form  a  water  jet,  which 
fmpinges  fairly  on  the  bucket  of  the  runner  without  causing  a 
uniform  pressure  either  throughout  the  bucket  spaces  or  in  the 
space  between  runner  and  guides.  It  is  not  intended  that  the 
passages  of  the  wheel  should  be,  as  in  the  pressure  turbine, 
entirely  filled  with  the  water,  nor  is  it  best  that  they  should 
be.  Fig.  201  gives  a  sectional  view  from  Unwin  showing  the 
arrangement  of  the  guide  blades  and  buckets  of  an  impulse 


Fio.  201. 

turbine,  in  which  the  flow  is,  as  in  the  pressure  turbine  previ- 
ously shown,  in  general  along  the  axis  of  the  wheel.  An 
impulse  turbine  necessarily  loses  all  the  head  below  the  wheel 
and  cannot  be  used  with  a  draft  tube. 

Occasionally  an  attempt  is  made,  in  the  so-called  limit  tur- 
bines, so  to  design  the  guides  and  buckets  that  the  jets  may 
completely  fill  the  buckets,  which  are  adapted  exactly  to  the 
shape  of  the  issuing  stream.  In  such  case  the  turbine  works 
as  an  impulse  wheel  or  as  a  pressure  wheel,  according  as  the 
draft  tube  is  or  is  not  used. 

A  modified  impulse  turbine,  largely  used  for  very  high  heads 
of  water,  is  found  in  the  Pel  ton  and  similar  wheels,  in  which 
the  impulse  principle  is  used  through  a  single  nozzle  acting  in 
succession  on  the  buckets  of  a  wheel  which  revolves  in  the 


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353 


same  plane  with  the  issuing  jet.  Such  a  Pelton  wheel  is  shown 
in  Fig.  202.  Occasionally  two  or  more  nozzles  are  used,  de- 
livering water  to  the  same  wheel.  Impulse  wheels  of  this  class 
are  exceedingly  simple  and  efficient,  and  work  admirably  on 
high  heads  of  water.  They  are,  moreover,  very  flexible  in  the 
matter  of  obtaining  efficiently  various  speeds  of  rotation  from 
the  same  head  of  water,  as  the  whole  structure  is  so  simple 
and  cheap  that  it  can  be  modified  easily  to  suit  varying  condi- 
tions. 

It  is  obvious  that  the  operation  of  such  an  impulse  wheel  is 
similar  to  that  of  a  true  impulse  turbine,  in  which  only  one, 


Fig.  202. 

or  at  the  most  three  or  four  jets  from  the  gmde  blades  are  util- 
ized. Most  of  the  hydraulic  work  done  in  this  country  is  ac- 
complished with  pressure  turbines,  which  are  worthy,  therefore, 
of  some  further  description.  A  small  but  important  por- 
tion is  accomplished  by  Pelton  and  other  impulse  wheels,  and 
in  a  very  few  instances  the  impulse  turbine  proper  has  been 
used. 

There  are  manufactured  in  this  country  more  than  a  score 
of  varieties  of  pressure  turbines.  They  differ  widely  in  de- 
sign and  general  arrangement,  but  speaking  broadly  it  is  safe 
to  say  that  most  of  them  are  of  the  mixed  discharge  type,  in 


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ELECTRIC  TRANSMISSION  OF  POWER. 


which  the  water  passes  away  from  the  buckets  of  the  runner 
inward  and  downward  with  reference  to  the  axis  of  the 
wheel.  It  would  be  impossible  to  describe  even  a  considerable 
part  of  them  without  making  a  long  and  useless  catalogue. 
The  essential  points  of  difference  are  generally  in  the  con- 
struction of  the  runner  and  in  the  mechanism  of  the  guide 
blades.  In  a  good  many  turbines  regulation  is  accomplished 
by  shifting  the  guide  blades  so  as  to  deliver  more  or  less  water 
to  the  runner.  A  few  types  will  serve  to  illustrate  the  general 
character  of  some  of  the  best-known  American  wheels.     Fig. 


FlOS.  203  AND  201. 

203  shows  the  so-called  Samson  turbine  of  James  Leffel  & 
Co.,  and  Fig.  204,  the  rimner  belonging  to  it.  This  wheel  is  of 
the  class  which  regulates  by  shifting  the  guide  blades,  which 
are  balanced  and  connected  to  the  governor  by  the  rods  at  the 
top  of  the  casing  shown.  The  water  enters  the  guide  blades  in- 
wardly, and  the  runner  is  provided  as  sho\Mi  with  two  sets  of 
buckets;  the  upper  set  discharging  inwardly,  the  lower  and 
larger  set  downwardly.  The  action  of  the  wheel  is  almost 
equivalent  to  two  wheels  on  the  same  shaft,  the  intention  being 
to  secure  an  unusually  large  power  and  speed  from  a  given 


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WA  TER-W HEELS. 


355 


head  of  water  on  a  single  wheel  structure.  This  result  is, 
as  might  be  anticipated,  accomplished,  and  for  a  given  diam- 
eter the  Samson  turbine  has  a  speed  and  power  consider- 
ably greater  for  a  given  head  than  found  in  the  usual  standard 
single  wheels.  As  before  remarked,  however,  it  is  almost, 
mechanically  speaking,  equivalent  to  two  wheels  through  its 
peculiar  feature  of  double  discharge  through  independent 
buckets. 

Another  very  excellent  and  well-known  wheel  is  the  Victor 
turbine,  shown  in  Fig.  205.     In  this  wheel  the  gate  is  of  the  so- 


Fia.  205. 

called  cylinder  type,  which  lengthens  or  shortens  the  apertures 
admitting  water  to  the  guide  blades.  The  runner  of  this 
wheel  is  so  shaped  that  the  water  is  discharged  inwardly 
and  downwardly.  The  area  of  the  rimner  blades  exposed 
to  the  full  water  pressure  is  notably  great.  The  cylinder 
form  of  gate  is  rather  a  favorite  with  American  wheel  man- 
ufacturers, and  is  intended  to  secure  a  somewhat  uniform 
efficiency  of  the  wheel,  both  at  full  and  part  load,  although 


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ELECTRIC  TttANSMlSStON  OF  POWER. 


how  completely  it  does  this  is  a  matter  which,  of  course,  is  still 
in  dispute.  The  wheel  shown,  however,  is  an  exceptionally 
good  and  efficient  one,  so  far  as  can  be  judged  from  general 
practice.  The  same  makers  also  manufacture  a  wheel  with 
shifting  guide  blades. 

Another  excellent  wheel  of  the  cylinder  gate  type,  the 
McCormick,  is  showTi  in  Fig.  206.  The  runner  of  this  wheel  has 
its  main  discharge  downward.  It  has  a  rather  large  power  for 
its  diameter,  owing  to  the  proportion  of  the  runners,  and  is 


Fig.  206. 

well  known  as  a  successful  wheel  considerably  used  in  driving 
electrical  machinery. 

These  turbines  are  typical  of  the  construction  and  arrange- 
ment used  by  first-class  American  manufacturers.  They  are 
all  arranged  for  either  horizontal  or  vertical  axes,  and  for 
purposes  of  driving  electrical  machinery  are  whenever  possible 
used  in  the  horizontal  form.  All  of  them,  particularly  the  two 
first  mentioned,  have  been  widely  used  for  electrical  piu'poses. 
They  are  all  practically  pure  pressure  turbines  and  are  installed 


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WATER-WHEELS. 


857 


usually  with  draft  tubes  of  appropriate  length.  They  are 
often,  too,  installed  in  pairs,  two  wheels  being  placed  on  the 
same  shaft,  fed  from  a  common  pipe  but  discharging  through 
separate  draft  tubes.  The  arrangement  of  these  draft  tubes 
is  very  various,  as  they  can  be  placed  in  any  position  convenient 
for  the  particular  work  in  hand.  Fig.  207  shows  a  common 
arrangement  where  a  single  wheel  is  to  be  driven.     The  water 


Fig.  207. 

enters  through  the  penstock,  passing  into  the  wheel  case, 
through  the  wheel,  which  has,  as  is  generally  the  case  except 
with  very  low  heads,  a  horizontal  axis,  and  thence  passes  into 
the  tail-race  through  the  draft  tube,  shown  in  the  lower  part  of 
the  cut.  The  full  head  in  the  particular  case  shown  is  43  feet, 
so  that  the  draft  tube  is  fairly  long.  Where  double  wheels  are 
employerl,  there  is  no  longer  any  necessity  of  taking  up  the 
longitudinal  thrust  of  the  wheel  shaft,  and  an  arrangement 


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358  ELECTRIC  TRANSMISSION  OF  POWER, 

frequently  followed  is  shown  in  Fig.  208,  which  gives  a  very 
good  idea  of  the  general  arrangement  of  the  pair  of  horizontal 
turbines,  which  may  be  directly  coupled  to  the  load  or,  as  in 
the  case  just  mentioned,  drive  it  through  the  medium  of  belts. 
In  many  instances  it  is  found  cheaper  and  simpler  to  moimt 
the  two  wheels  together  in  a  single  flume  or  wheel-case,  so  as 
to  discharge  into  the  same  draft  tube.  Fig.  209  shows  an 
arrangement  which  is  thoroughly  typical  of  this  practice, 
applied  in  this  case  to  a  low  head.  The  pair  of  wheels  are 
here  arranged  so  as  to  discharge  into  a  common  draft  tube 


Fio.  208 

between  them,  while  they  receive  their  water  from  the  timber 
penstock  in  which  they  are  inclosed.  Such  wooden  penstocks 
are  generally  very  much  cheaper  than  iron  ones  and  for  low 
heads  have  been  extensively  used. 

The  central  draft  tube  here  shown  need  not  go  vertically 
downwards,  but  may  take  any  direction  that  the  arrangement 
of  the  tail-race  requires.  Whether  the  draft  tube  is  single  or 
double  is  determined  mainly  by  convenience  in  arranging  the 
wheel  and  its  foundations,  and  the  tail-race.  The  use  of  a  pair 
of  turbines  coupled  together  is  not  only  important  in  avoiding 


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WATER-WHEELS, 


359 


end  thrust,  but  it  also  enables  a  fair  rotative  speed  to  be 
obtained  from  moderate  heads,  which  is  sometimes  very  im- 
portant in  driving  electrical  machinery. 

For  example,  suppose  one  desired  to  drive  a  500  KW  gener- 
ator by  turbines  from  a  25  foot  head.  Allowing  a  little  margin 
for  overload  the  turbine  capacity  should  be  in  the  neighbor- 
hood of  750  HP.  Now  turning  to  a  wheel  table  applying,  for 
instance,  to  the  "Victor"  wheel,  one  finds  that  a  single  54" 
wheel  would  do  the  work,  but  at  the  inconveniently  low  speed 
of  128  r.p.m.     But  under  the  same  head  a  pair  of  39^  wheels 


Fig.  209. 

would  give  a  little  larger  margin  of  power  at  180  r.p.m.,  and 
hence  would  probably  enable  one  to  get  his  dynamo  at  lower 
cost,  as  well  as  to  avoid  a  thrust  bearing.  Often  such  a  change 
of  plan  will  allow  the  use  of  a  standard  generator  where  a 
special  one  would  otherwise  be  necessary. 

Wherever  possible  it  is  highly  desirable  to  employ  these  hori- 
zontal wheels  for  electrical  purposes,  inasmuch  as  power  has,  in 
most  cases,  to  be  transferred  to  a  horizontal  axis,  and  the  use 
of  a  vertical  shaft  wheel  necessitates  some  complication  and 
loss  of  power  in  changing  the  direction  of  the  motion.  Occa- 
sionally a  vertical  shaft  wheel  is  used  for  electrical  purposes, 
driving  a  dynamo  having  a  vertical  armature  shaft.    This  prac- 


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360  ELECTRIC  TRANSMISSION  OF  POWER, 

tice  is  not  generally  to  be  recommended,  as  it  involves  special 
dynamos,  and  a  somewhat  troublesome  mechanical  problem 
in  supporting  the  weight  of  the  armature,  which  is  generally 
carried  by  hydraulic  pressure.  A  fine  example  of  this  arrange- 
ment is  to  be  found  in  the  great  Niagara  Falls  plant. 

The  use  of  pressure  turbines  for  driving  electrical  machinery 
is  exceedingly  convenient  on  low  or  moderate  heads,  say  up  to 
50  or  100  feet.  With  higher  heads  frequently  the  rotative  speed 
becomes  inconveniently  great;  for  example,  under  100  feet 
head,  150  HP  can  be  obtained  from  a  wheel  a  little  more  than 
15  inches  in  diameter,  at  a  speed  of  more  than  1,000  revolu- 
tions per  minute.  At  200  feet  head,  the  power  for  the  same 
wheel  will  have  risen  to  about  400  HP  and  the  speed  to  nearly 
1,300.  This  is  a  rather  inconvenient  speed  for  so  large  a  power, 
and  it  is  necessitated  by  the  fact  that  a  pressure  turbine  to 
work  under  its  best  conditions  as  to  efficiency,  must  rim  at 
a  peripheral  speed  of  very  nearly  three-quarters  the  full 
velocity  of  water  due  to  the  head  in  question.  If,  therefore, 
turbines  are  used  for  high  heads,  either  the  dynamos  to 
which  they  are  coupled  must  be  of  decidedly  abnormal  design, 
or  the  dynamo  must  be  run  at  less  speed  than  the  wheels. 

The  former  horn  of  the  dilemma  was  taken  in  the  Niagara 
plant,  and  involved  some  very  embarrassing  mechanical  ques- 
tions in  the  construction  of  the  djmamos.  Where  belts  are 
permissible  the  other  practice  is  the  more  usual,  of  which  a 
good  example  is  found  in  the  large  lighting  plant  at  Spokane 
Falls,  Wash.,  where  the  wheels  were  belted  to  the  dynamos 
for  a  reduction  in  speed  instead  of  an  increase,  as  is  usually 
the  case. 

Impulse  turbines  are  little  used,  although  manufactured 
to  some  extent  by  the  Girard  Water  Wheel  Co.,  of  San  Fran- 
cisco, Cal.  The  wheel  manufactured  by  them  is  one  with  a  well- 
known  foreign  reputation.  Its  general  arrangement  is  well 
shown  by  the  diagram,  Fig.  210.  The  Girard  impulse  turbine  is 
of  the  outward  flow  type,  a  form  rather  rare  in  pressure  tur- 
bines. The  water  enters  the  wheel  centrally  through  a  set  of 
guide  blades,  which  form  a  series  of  nozzles  from  which  the 
water  issues  with  its  full  spouting  velocity  and  impinges  on  the 
buckets   of   the   nuiner,    which   siu'romids   the  guide   blades. 


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WATEEr-WHEELS.  861 

The  discharge  is  virtually  radially  outward.  Regulation  is 
secured  by  a  governor  which  either  cuts  ofif  one  or  more  of  the 
nozzles  or  may  be  arranged  by  swinging  guide  blades  to  con- 
tract all  or  a  part  of  the  nozzles.  In  either  case,  there  is  no 
water  wasted,  and  the  wheel  works  efficiently  at  practically  all 
loads. 

Like  others  of  the  impulse  type,  the  peripheral  speed  of  the 
wheel  when  worked  \mder  its  best  conditions  for  efficiency,  is 
very  nearly  one-half  the  spouting  velocity  of  the  water  as  it 


Fio.  210. 

issues  from  the  nozzle.  This  produces  for  a  wheel  of  given 
diameter  a  lower  speed  for  the  same  head  than  in  the  case  of 
pressure  turbines,  while  the  use  of  a  larger  number  of  nozzles 
working  simultaneously  on  the  runner  gives  a  higher  power  for 
the  same  diameter  than  in  the  case  of  the  Pelton  or  similar 
wheels,  which  use  only  a  few  nozzles  with  jets  applied  tan- 
gentially;  hence,  such  impulse  turbines  occupy  a  useful  place 
in  the  matter  of  speed,  aside  from  all  questions  of  efficiency. 

Under  moderately  high  heads,  from  100  up  to  300  or  400 
feet,  they  give  a  much  greater  power  for  a  given  rotative 


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862  ELECTRIC  TRANSMISSION  OF  POWER. 

speed  than  impulse  wheels  emplo3dng  only  two  or  three 
nozzles.  On  the  other  hand  they  do  not  run  inconveniently 
fast,  as  is  the  case  with  pressure  turbines  under  such  heads. 
At  extremely  high  heads  they  give,  unless  operated  with 
only  one  or  a  few  nozzles,  so  great  power  as  to  be  inconve- 
nient for  the  high  speed  attained,  so  far  at  least  as  the  oper- 
ation of  dynamos  is  concerned.  At  very  low  heads  there 
is  material  loss  from  the  fact  that  the  wheel  cannot  be  used 
with  the  draft  tube,  and  consequently  a  certain  amount  of  the 
head  must  be  sacrificed  to  secure  free  space  from  the  wheel  to 
the  tail-water.  These  Girard  turbines  are  made  with  both 
vertical  and  horizontal  axes,  and  are  applicable  to  electrical 
work  with  the  same  general  facility  which  applies  to  other 
types  of  wheel.  Their  strong  point  is  economical  and  effi- 
cient regulation  of  the  water  supply,  together  with  high  effi- 
ciency at  moderate  loads. 

The  Pelton  wheel,  already  shown  in  Fig.  202,  may  be  regarded 
as  an  impulse  turbine  having  a  single  nozzle,  and  that  applied 
tangentially.  These  wheels  have  proved  immensely  effective 
for  heads  from  several  hundred  up  to  a  couple  of  thousand  feet. 
Like  the  true  impulse  turbines,  the  peripheral  speed  should  be 
half  the  spouting  velocity  of  the  water,  hence,  by  varying  the 
dimensions  of  the  wheel  a  wide  range  of  speed  can  be  obtained, 
which  is  exceedingly  convenient  in  power  transmission  work, 
permitting  direct  coupling  of  the  dynamos  under  all  sorts  of 
conditions.  They  are  not  infrequently  made  with  two  or  three 
nozzles,  which  give,  of  course,  correspondingly  greater  power 
for  the  same  speed.  At  heads  of  only  100  or  200  feet  these 
wheels  with  their  few  nozzles  give  "an  inconveniently  low  rota- 
tive speed  for  the  power  developed,  and  are  at  their  best  in 
this  respect  between  300  and  1,000  feet.  The  Pelton  wheel 
is  usually  regulated  by  deflecting  the  nozzles  away  from  the 
buckets  of  the  wheel,  a  very  effective  but  most  inefficient 
method,  so  far  as  economy  of  water  is  concerned.  The  wheel 
has,  however,  under  favorable  conditions,  a  very  high  efficiency, 
certainly  as  high  as  can  be  reached  with  any  other  form  of 
hydraulic  prime  mover.  The  practical  results  given  by  this 
class  of  wheel  are  admirable  under  circumstances  favorable  to 
their  use,  and  the  Pelton  and  Doble  wheels  have  played  a  very 


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WATER-WHEELS.  868 

important  part  in  the  great  power  transmission  works  which 
have  placed  the  Pacific  coast  in  the  van  of  modem  engineering. 

A  recent  improvement  in  impulse  wheel  practice  is  the 
development  of  a  successful  "needle  valve"  for  the  nozzles^ 
which  obviates  the  waste  of  water  due  to  the  use  of  deflecting 
nozzles.  The  needle  valve  is  simply  a  nozzle  which  can  be 
closed  at  will  by  a  central  plunger,  moving  axially  in  the 
stream  just  behind  the  nozzle.  The  plunger  and  its  seat  are 
given  surfaces  curved  in  such  wise  that  in  all  positions  of  the 
plunger  a  smooth  emergent  stream  is  produced,  and  the  effi- 
ciency of  the  wheel  is  very  Uttle  changed. 

This  is  upon  the  whole,  a  better  method  of  regulation  than 
the  deflecting  nozzle  in  that  it  is  economical  of  water,  but 
shutting  off  the  stream  quickly  produces  very  severe  strains 
in  the  pipe  line  and  in  most  instances  some  form  of  relief  valve 
is  desirable  to  reduce  the  pressure.  To  use  any  form  of  nozzle 
valve,  too,  the  water  must  be  thoroughly  freed  from  sand, 
which  at  the  stream  velocities  often  used,  cuts  even  the  toughest 
metal  with  great  rapidity. 

Another  wheel  of  this  class  is  the  Leffel  "Cascade"  water- 
wheel.  Two  complete  rings  of  buckets  are  employed  for  this 
wheel,  and  the  wheels  are  arranged  to  be  supplied  from  several 
nozzles,  of  which  one  or  more  are  put  into  use  according  to 
the  necessities  of  regulation.  The  cascade  wheel  therefore 
occupies  a  place,  as  it  were,  between  the  ordinary  impulse 
wheel  and  the  impulse  turbine,  resembling  the  former  in  the 
arrangement  of  its  multiple  jets,  and  the  latter  in  the  method 
of  regulation  by  cutting  off  completely  some  of  the  nozzles. 

From  the  foregoing  it  will  be  appreciated  that  each  of  the 
three  general  classes  of  wheels  described,  pressure  turbines, 
impulse  turbines,  and  tangential  impulse  wheels,  has  a  sphere 
of  usefulness  in  which  it  can  hardly  be  approached  by  either 
of  the  others.  It  is  worth  while,  therefore,  to  examine  some- 
what in  detail  the  conditions  of  economy  under  various  cir- 
cumstances. 

The  pressure  turbine  has  its  best  field  under  relatively  low 
and  imiform  heads.  By  means  of  the  draft  tube  no  head  is 
lost,  as  is  the  case  with  that  portion  of  the  head  which  lies 
between  the  turbine  and  the  tail-water  in  the  use  of  impulse 


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ELECTRIC  TRANSMISSION  OF  POWER. 


wheels  of  any  description.  Further,  the  pressure  turbine  under 
all  heads  gives  a  higher  relative  speed  than  the  impulse  wheels, 
whether  of  the  tangential  or  turbine  variety,  and  under  low 
heads  is  apt  to  be  of  less  bulk  and  cost  and  to  give  a  more  con- 
venient speed  for  electric  work;  hence,  these  pressure  turbines 
have  been  more  extensively  used  than  any  other  variety  of 
water-wheels  in  the  enormous  hydraulic  developments  of  the 
last  quarter  of  a  century.  Furthermore,  the  pressiu^  turbine 
has,  under  favorable  conditions,  as  high  efficiency  as  any  known 


.7  .8 

PROPORTIONAL  DI8CHARQE 
Fio.  211. 


JT 


variety  of  water-wheel.     The  losses  of  energy  are  mainly  of 
four  kinds. 

1.  Friction  of  bearings,  usually  small. 

2.  Friction  and  eddying  in  the  wheel  and  guide  passages. 

3.  Leakage,  and 

4.  Unutilized  energy  of  other  kinds,  largely  owing  to  imper- 
fect shaping  of  the  working  parts,  or  loss  of  head. 

With  the  best  construction  these  losses  aggregate  15  to 
20  per  cent.  Of  them  the  shaft  friction  is  the  smallest  and 
the  loss  from  friction  and  eddies  in  the  wheel  the  largest, 
probably  fully  half  of  the  total  loss,  particularly  under  high 


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WATER-WHEELS.  865 

heads.  This  efficiency  is  approximately  true  of  the  better 
class  of  turbines,  whether  of  the  pressure  or  impulse  variety. 
Under  low  and  uniform  heads  the  pressure  turbine  probably  is 
capable  of  a  little  better  work  than  the  impulse  variety,  but  both 
suffer  if  the  head  varies.  The  curves,  Fig.  211,  show  efficiency 
tests  made  with  the  greatest  care  on  four  first-class  pressure  tur- 
bines at  the  Holyoke  testing  flume,  probably  the  best  equipped 
place  in  the  world  for  making  such  tests.  It  should  be  noted 
that  the  efficiency  of  all  the  wheels  shown  is  good;  over  80 
per  cent  at  full  admission  of  water;  at  partial  admission  the 
efficiencies  vary  more  between  the  individual  wheels.  This 
variation  is  largely  due  to  the  methods  of  regulating  the  flow 
employed.    These  are  in  general  three: 

1.  Varying  the  number  of  guide  passages  in  use. 

2.  Varying  the  area  of  these  guide  passages  by  moving  the 
guide  blades. 

3.  Var3ang  the  admission  to  the  guide  passages  by  a  gate 
covering  the  entrance  to  all  of  them. 

The  first  method  is  particularly  bad,  as  the  buckets  are  at  one 
moment  exposed  to  full  water  pressure  and  then  come  opposite 
a  closed  passage,  setting  up  a  good  deal  of  unnecessary  shock 
and  eddying.  It  is  a  method  that  is  scarcely  ever  used  in  this 
country.  Between  the  other  two  it  is  not  so  easy  to  choose. 
Both  have  strong  advocates  among  wheel  makers;  some  com- 
panies building  both  types,  and  the  others  only  one  of  them. 
The  curves  shown  represent  both  these  methods  of  regulation. 

The  truth  probably  is  that  the  relative  efficiency  of  the  two 
depends  more  on  the  design  of  the  wheel  with  reference  to  its 
particular  form  of  regulation,  than  on  the  intrinsic  advantages 
of  either  form.  Turbines  are  generally  constructed  so  that 
the  point  of  maximum  efficiency  is  rather  below  the  maxi- 
mum output,  as  a  little  leeway  is  desired  for  purposes  of  regu- 
lation under  varying  heads,  so  that  the  design  is  arranged  to 
give  the  best  efficiency  of  which  the  wheel  is  capable  at  a  point 
a  little  below  full  admission.  These  efficiency  curves  were 
taken  at  heads  of  from  15  to  18  feet  and  show  what  can  be  regu- 
larly accompUshed  by  good  wheel  design.  They  are  neither 
phenomenally  high  nor  unusually  low.  Occasionally  efficiencies 
are  recorded  slightly  better  than  those  shown.    In  this  conneo- 


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ELECTRIC  TRANSMISSION  OF  POWER. 

tion  it  is  desirable  to  state  in  the  way  of  warning,  that  there 
was  obtained  at  the  Holyoke  flume  some  years  ago  a  series 
of  tests  of  turbines  of  more  than  one  make,  which  showed 
enormously  high  efficiency,  afterward  traced  to  a  constant 
error  in  the  experiments.  As  the  fact  of  the  error  was  not 
so  generally  known  as  the  result  of  the  tests,  occasionally 
reports  are  heard  of  phenomenal  turbine  efficiencies  which 
are  given  in  entire  good  faith,  but  based  on  errors  of  experi- 
ment. It  is  only  fair  to  say  that  the  tests  now  made  at  the 
Holyoke  flume  are  worthy  of  entire  confidence. 

As  regards  impulse  turbines,  data  are  hard  to  obtain. 
Those  which  are  available  indicate,  however,  that  with  an  effi- 
ciency probably  a  little  less  at  full  load  than  that  of  pressure 
turbines  under  moderate  heads,  the  half-load  efficiency  is 
generally  considerably  higher.  This  is  owing  to  the  fact  that 
the  buckets  of  the  rimner  work  entirely  independently  of  each 
other,  and  the  water  acts  in  precisely  the  same  way  on  each 
bucket  whether  it  is  received  from  all  the  nozzles  formed  by  the 
guide  blades,  or  from  a  part  of  them.  The  impulse  turbuies  are 
generally  regulated  by  cutting  off  more  or  less  of  the  nozzles. 
The  shaping  of  the  surfaces  in  the  runner  and  guide  blades, 
and  the  smoothness  of  the  finish,  are  of  more  importance 
in  these  wheels  than  in  the  ordinary  pressure  turbines.  The 
impulse  turbines  are,  as  has  already  been  stated,  peculiarly 
adapted  in  point  of  speed  and  general  characteristics  for  use 
on  moderately  high  heads,  and  in  this  work  they  give  a  better 
average  efficiency  and  more  economical  use  of  water  than  any 
of  the  pressure  turbines.  For  low  heads  their  advantages  are 
far  less  marked,  and  the  pressure  turbines  are  generally 
preferred. 

The  tangential  impulse  wheels  are,  at  full  admission  of  water, 
of  an  efficiency  quite  equal  to  that  of  the  best  turbines.  At 
partial  admission  they  cannot  be  expected  to  give  the  same 
results  as  do  the  best  impulse  turbines,  inasmuch  as  they  regu- 
late generally  by  deflecting  the  nozzle  away  from  the  buckets, 
and  hence  wasting  water.  The  variation  of  the  stream  by  a 
needle  valve  considerably  relieves  this  difficulty  in  cases  where 
it  can  be  successfully  applied.  For  very  high  heads,  however, 
the  tangential  wheels  are  preferable  to  any  turbines,  as  they 


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WATER-WHEELS,  867 

give  a  better  relation  between  power  and  speed,  so  far  as  driv- 
ing electrical  machinery  is  concerned,  and  their  extreme 
simplicity  is  favorable  to  good  continuous  working  under  the 
enormous  strains  produced  by  the  impact  of  water  at  great 
spouting  velocities. 

To  summarize,  pressure  turbines  are  admirable  for  low  and 
uniform  heads,  particularly  where  the  load  is  steady.  The 
impulse  turbines  give  more  efficient  use  of  water  at  part  load, 
and  a  more  convenient  speed  on  moderately  high  heads.  The 
tangential  impulse  wheels  do  relatively  the  best  work  under 
very  high  heads,  and  where  water  does  not  have  to  be  rigidly 
economized.  Each  of  the  three  classes  has  decided  advantages 
over  the  others  in  particular  situations,  and  the  full  load  effi- 
ciency of  all  three  is  approximately  equal.  The  choice  of 
either  one  of  these  types  should  be  made  in  each  individual 
case  in  accordance  with  the  hydraulic  conditions  which  are  to 
be  met.  The  choice  between  particular  forms  of  each  type  is 
largely  a  commercial  matter,  in  which  price,  guarantees, 
facility  of  getting  at  the  makers  in  case  of  repairs,  standard 
sizes  fitting  the  particular  case  in  hand,  and  similar  considera- 
tions are  likely  to  determine  the  particular  make  employed, 
rather  than  any  broad  difference  in  construction  or  operation. 

The  success  of  a  power  transmission  plant  depends  quite  as 
much  on  careful  hydraulic  work  as  on  proper  electrical  instal- 
lation. The  two  should  go  hand  in  hand,  and  any  attempt, 
such  as  is  often  made,  to  contract  for  the  two  parts  of  the 
plant  independently  of  each  other,  or  to  engineer  them  inde- 
pendently, generally  results  in  a  combination  of  electrical  and 
hydraulic  machinery  that  is  far  from  being  the  best  possible 
under  the  conditions,  and  is  quite  likely  to  be  anything  but 
satisfactory. 

The  hydraulic  and  electrical  engineers  should  go  over 
the  arrangement  of  the  plant  together  with  a  view  to  adapting 
each  class  of  machinery  to  the  other  as  perfectly  as  possi- 
ble, in  order  to  get  a  symmetrical  whole.  Many  troublesome 
questions  have  to  be  encountered,  and  only  the  closest  study 
will  lead  to  perfectly  successful  results. 

One  of  the  commonest  and  most  serious  difficulties  met  with 
in  la3dng  out  an  electrical  and  hydraulic  plant  for  transmission 


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368  ELECTRIC  TRANSMISSION  OF  POWER, 

work,  lies  in  the  variability  of  the  head  of  water.  There  are 
comparatively  few  streams  from  which  can  be  obtained  an  in- 
variable head  practically  independent  of  low  water  or  freshets. 
The  usual  condition  of  things  is  to  find  a  fairiy  uniform  head 
for  nine  or  ten  months  in  the  year,  and  rather  wide  variations 
during  the  remainder  of  the  time.  It  is  not  at  all  uncommon  to 
meet  a  water-power  which,  even  when  very  skilfully  developed, 
will  stiU  entail  upon  the  user  a  variation  of  25  or  30  per  cent 
in  the  available  head. 

At  the  time  of  high  water  this  appears  as  a  rise  of  water 
level  in  the  tail-race,  so  as  to  diminish  the  head  available  for  the 
wheels.  In  times  of  low  water,  the  head  might  be  normal,  but 
the  quantity  of  water  altogether  insufficient.  Any  variations 
of  this  kind  are  of  a  very  serious  character,  because  they  not 
only  vary  the  amount  of  power  which  is  available,  but  they 
change  the  speed  of  the  wheels  so  that  the  dynamos  no  longer 
wiU  operate  at  their  proper  speed  and  hence  will  change  in 
voltage,  and  if  alternating  apparatus  is  used,  in  frequency  also, 
which  is  even  more  serious.  For  example:  Under  24  feet 
head  one  of  the  well-known  standard  wheels  gives  nearly 
650  HP  at  100  revolutions  per  minute.  Under  16  feet  head 
the  same  wheel  would  give  only  352  HP  at  82  revolutions. 

The  lack  of  power  occurring  at  the  time  of  high  water  is 
serious.  The  change  of  speed,  although  not  great,  is  very 
annoying,  and  should  be  avoided  if  possible.  Changes  much 
greater  than  this  are  common  enough.  The  season  of  reduced 
head  is  generally  short,  not  over  a  couple  of  months,  often 
only  a  week  or  two,  and  this  renders  the  situation  doubly 
embarrassing,  because  during  a  large  part  of  the  year  the  same 
wheel  must  be  able  to  operate  economically.  The  methods 
taken  to  get  out  of  this  difficulty  of  varying  head  are  various; 
most  of  them  bad.  One  of  the  commonest  is  to  arrange  the 
wheels  to  operate  normally  at  partial  gate,  then  on  the  low 
heads  to  throw  the  gate  wide  open  and  obtain  increased  power. 
On  the  high  heads  the  wheel  is  throttled  still  more.  Such  an 
arrangement  works  the  dynamo  in  a  fairly  efficient  fashion, 
but  the  wheel,  as  a  rule,  quite  inefficiently  a  large  portion  of 
the  time,  as  may  be  seen  by  reference  to  the  efficiency  curves 
of  the  wheels  just  given.     It  is  a  practice  similar  to  that  which 


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WATER-WHEELS.  869 

one  would  find  in  working  an  engine  at  part  load.  For  moder- 
ate variations  of  head,  not  exceeding  10  per  cent,  the  loss  of 
efficiency  is  not  so  serious  as  to  bar  this  very  simple  plan,  but 
under  conditions  too  frequently  encountered,  these  variations 
of  efficiency  would  be  so  great  as  to  make  the  method  exceed- 
ingly undesirable. 

Hydraulic  plants  are  occasionally  operated  without  any 
reference  to  economy  of  water,  and  in  such  cases  the  practice 
of  operating  normally  at  part  gate  is .  frequently  followed, 
but  it  must  be  remembered  that  as  water  powers  are  more 
and  more  developed,  economy  of  water  becomes  more  and 
more  necessary,  and  in  every  case  should  be  borne  in  mind 
even  if  it  is  not  rendered  necessary  by  conditions  actually 
existing.  In  thoroughly  developed  streams  it  is  generally 
important  to  waste  no  water. 

Another  method  of  overcoming  the  difficulties  due  to 
variations  of  head,  is  the  installation  of  two  wheels  on  the 
same  shaft,  one  intended  to  give  normal  power  and  speed  at 
the  ordinary  head,  the  other  at  the  emergency  head.  This 
practice  is  carried  out  in  variou?  forms.  Sometimes  two 
wheels  may  be  moimted  on  the  same  horizontal  or  vertical 
axis,  and  one  of  them  is  disconnected  or  permitted  to  run 
idle  except  when  actually  needed.  Another  modification  of 
the  same  general  idea  is  the  use  of  a  duplex  wheel  with  the 
runner  and  guides  arranged  in  two  or  three  concentric  sets  of 
buckets,  which  can  be  used  singly  or  together  according  to 
the  head  which  is  available. 

A  fine  example  of  this  practice  is  found  in  the  great  power 
plant  at  Geneva,  to  which  reference  has  already  been  made, 
where  the  head  varies  from  5^  to  12  feet.  Here  the  turbines 
have  buckets  arranged  in  three  concentric  rings,  the  outermost 
being  used  at  the  highest  head  and  all  three  at  the  lowest  head. 
Under  the  latter  condition,  the  average  radius  at  which  the 
water  acts  upon  the  wheel  is  diminished  and  the  speed  is 
therefore  increased,  while  the  greater  volume  of  water  keeps 
up  the  power.  The  various  combinations  possible  with  the 
rings  of  buckets  are  so  effective  in  keeping  the  speed  imiform 
that  the  extreme  variation  of  speed  under  the  maximum  varia- 
tion of  head  is  only  about  10  per  cent.     Such  a  triplex  turbine 


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S70  ELECTRIC  TRANSMISSION  OF  POWER. 

is  of  high  first  cost,  but  is  decidedly  economical  of  water  at 
normal  load.  Still  another  variation  of  the  double  turbine 
idea  consists  of  installing  two  turbines  for  each  unit  of  power, 
one  acting  directly,  the  second  through  the  medium  of  belts. 
The  direct-acting  turbine  is  intended  for  normal  load,  the 
belted  turbine  of  larger  dimensions  for  use  during  the  periods 
of  low  head. 

This  arrangement  is  used  in  the  large  power  transmission 
plant  at  Oregon  City,  Ore.  It  is  economical  of  water,  but  is 
mechanically  somewhat  complicated.  It  is  probably  on  the 
whole  less  desirable  than  the  installation  of  two  turbines  on 
the  same  shaft,  and  much  less  desirable  than  the  duplex  or 
triplex  arrangement  just  referred  to.  Where  two  turbines  are 
operated  on  the  same  shaft,  it  is  generally  possible  to  arrange 
the  turbine  designed  to  operate  on  the  lower  head  so  as  to  run 
at  a  disproportionately  high  velocity  with  some  loss  of  effi- 
ciency, and  so  to  hold  the  speed  fairly  uniform. 

Still  another  method  of  counteracting  the  variation  of 
head  is  applicable  only  where  the  power  is  transmitted  from 
the  turbine  by  gears  or  belts.  In  this  case  it  is  always  possi- 
ble to  operate  the  machinery  under  the  reduced  head  with 
some  loss  of  output,  but  still  at  or  near  the  proper  speed. 
Whatever  way  out  of  the  difficulty  is  chosen,  it  should  be 
borne  in  mind  that  the  most  desirable,  on  the  whole,  is  the 
one  which  will  work  the  wheels  during  the  generally  long 
period  of  fairly  steady  head  at  their  best  efficiency.  If  there 
is  to  be  any  sacrifice  of  efficiency,  it  should  by  all  means  be 
for  as  short  a  time  as  possible,  and,  therefore,  should  be  at 
the  periods  of  extreme  low  head.  At  such  times  water  is 
generally  plenty,  while  at  the  higher  heads  economy  in  its 
use  is  more  necessary. 

REGULATION    OF   WATER- WHEELS. 

For  many  years  there  have  been  bad  water-wheel  governors 
and  worse  water-wheel  governors,  but  only  recently  have  there 
appeared  governors  which  may  be  classified  as  good  from  the 
standpoint  of  the  electrical  engineer.  It  has  been  necessary 
to  go  through  the  same  tedious  period  of  waiting  and  experi- 


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WATER-WHEELS.  S71 

mentation  that  was  encountered  before  dynamo  builders  could 
find  engines  which  would  hold  their  speed  at  var3dng  loads. 
Until  the  advent  of  electrical  transmission  work,  water-wheels 
were  most  generally  employed  for  certain  classes  of  manufac- 
turing, such  as  textile  mills,  where  the  speed  must  be  quite 
uniform,  but  where  at  the  same  time  the  load  is  almost  imiform; 
or,  on  the  other  hand,  for  saw  mills  and  the  like,  where  constant 
speed  is  of  no  particular  importance. 

The  action  of  water-wheel  governors  as  regards  the  way  in 
which  they  vary  the  supply  of  water  is  very  different;  some 
merely  act  to  open  or  close  the  head  gates;  others  to  work  a 
cylinder  gate  immediately  around  the  wheel,  and  still  others 
to  vary  the  area  of  the  guide  passages,  as  in  the  so-called 
register  gate  turbines. 

In  whatever  way  the  governing  action  takes  place,  its 
result  is  too  often  unsatisfactory,  due  to  the  great  difficulty 
that  has  to  be  encountered  in  the  great  inertia  of  the  water 
and  of  the  moving  parts  of  the  wheel.  Both  water  and  wheel 
are  sluggish  in  their  action,  and  as  a  result  some  time  elapses 
after  the  governor  has  produced  a  change  of  gate,  before 
that  change  becomes  effective.  Meanwhile,  the  speed  has 
fallen  or  risen  to  a  very  considerable  extent,  and  perhaps  in 
addition  the  load  has  again  changed  so  that  by  the  time  the 
speed  of  the  wheel  has  been  sensibly  affected  by  the  governor, 
the  direction  of  the  governing  action  may  be  exactly  opposite 
to  that  which  at  the  moment  is  desirable.  Even  if  this  is  not 
the  case,  the  governing  is  usually  carried  too  far,  being  con- 
tinued up  to  the  time  at  which  the  wheel  is  affected  and  reacts 
on  the  governing  apparatus,  hence  another  motion  of  the 
governor  becomes  necessary  to  counteract  the  excess  of  dili- 
gence on  the  part  of  the  first  action.  In  other  words,  the 
governor  "hunts,"  causing  a  slow  oscillation  of  the  speed 
about  the  desired  point,  an  oscillation  of  decreasing  amplitude 
only  if  the  new  load  on  the  wheel  be  steady. 

This  sluggishness  of  reaction  to  changes  indicated  by  the 
governor  is  the  most  formidable  obstacle  to  the  proper  control 
of  the  water-wheels.  To  overcome  it,  even  in  part,  it  is 
necessary  that  the  movement  of  the  gates  be  comparatively 
active,  if  the  changes  of  load  are  frequent,  and  this  entails  still 


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872  ELECTRIC  TRANSMISSION  OF  POWER. 

further  difficulty  by  caiLsing  severe  strains  on  the  mechanism 
and  the  gates,  particularly  if  the  water  is  led  to  the  turbine 
through  a  long  penstock.  In  the  latter  case,  the  variations  in 
pressure  produced  by  rapid  governing  are  often  dangerous, 
and  have  to  be  counteracted  by  air  chambers,  stand  pipes,  or 
the  like,  and  aside  from  all  this  there  is  a  still  further  difficulty 
in  the  considerable  weight  of  the  gate  and  the  pressure  against 
which  they  have  to  be  operated,  so  that  the  amount  of  mechani- 
cal power  controlled  by  the  governor  must  be  very  consider- 
able. 

A  very  large  variety  of  governors  have  been  designed  to 
meet  the  serious  difficulties  just  set  forth.  Most  of  them 
have  been  abject  failures,  aud  those  that  may  be  really  reck- 
oned of  some  considerable  value  for  electrical  work  may  be 
counted  on  the  fingers  of  one  hand. 

Water-wheel  governors  may  be  roughly  divided  into  two 
classes.  First,  come  those  regulators  in  which  the  wheel  itself 
supplies  power  to  the  gate-shifting  mechanism,  which  is  con- 
trolled by  a  fly  ball  governor  through  more  or  less  direct 
mechanical  means.  Second,  comes  the  relay  class  of  governors, 
wherein  all  the  work  possible  is  taken  off  the  centrifugal 
governor,  and  its  function  is  reduced  to  throwing  into  action 
a  mechanism  for  moving  the  gates  which  may  be  quite  inde- 
pendent of  any  power  transmitted  from  the  wheel  to  the  gov- 
erning mechanism.  The  various  classes  of  hydraulic,  pneu- 
matic, and  electric  governors  are  worked  in  this  way.  Their 
general  characteristic  is  that  their  sole  function  in  governing 
is  to  work  the  devices  which  control  the  secondary  mechanism, 
which  consists,  in  various  cases,  of  hydraulic  cylinders  oper- 
ating the  gates,  pneumatic  cylinders  serving  the  same  pur- 
pose, or  electric  motors  which  open  or  close  the  gates  by 
power  derived  from  the  machines  operated  by  the  turbines. 

A  vast  amoimt  of  ingenuity  has  been  spent  in  trying  to 
work  out  regulators  of  the  first  mentioned  class.  Almost 
every  possible  variety  of  mechanism  has  been  employed 
to  enable  the  governor  to  apply  the  necessary  power  to  the 
mechanism  operating  the  gates.  The  general  form  of  most  of 
these  governors  is  as  follows:  Power  is  taken  from  the  wheel 
shaft  by  a  belt  to  the  governor  mechanism,  where  it  serves  at 


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WATER-WHEELS.  873 

once  to  drive  the  governor  balls,  and  to  work  the  gates  when 
the  governor  connects  the  gate-controlling  gears  to  the  pulley 
whicji  supplies  the  power.  This  is  generally  done  by  friction 
cones  or  their  equivalent,  thrown  into  action  in  one  direction 
when  the  governor  balls  rise,  and  in  the  other  direction  when 
they  fall. 

Sometimes  this  mechanism  is  varied  by  employing  a  pair 
of  oscillating  dogs,  one  or  the  other  of  which  is  thrown 
into  appropriate  gearing  by  the  governors.  There  are  many 
governors  of  this  kind  on  the  market,  and  where  the  load  is 
fairly  steady  and  no  particular  accuracy  of  regulation  is  neces- 
sary, they  have  given  good  satisfaction.  The  fault  with  all 
governors  of  this  sort  is  that  the  centrifugal  balls  either  lack 
sensitiveness  or  lack  power.  If  the  governor  works  at  all 
rapidly  in  moving  the  gates,  too  heavy  a  load  is  tlu*owii  on 
the  governor  for  any  but  a  massive  mechanism,  and  the  cen- 
trifugal device  becomes  insensitive;  or,  on  the  other  hand,  if 
the  gates  are  worked  slowly,  the  governor  in  itself  is  sluggish 
and  ineffectual. 

In  most  cases  the  gates  are  made  to  move  quite  slowly. 
In  the  attempt  to  get  sensitiveness,  the  friction  wheels  or 
dogs  are  often  adjusted  so  closely  that  the  governor  is  in  a 
constant  slight  oscillatory  motion,  but  when  its  action  is 
really  needed,  as  in  the  case  of  a  sudden  change  of  load, 
response  generally  does  not  come  quickly  enough.  It  is  of 
course  possible  to  construct  a  mechanical  relay  which  would 
possess  both  power  and  sensitiveness,  but  nearly  all  the 
governors  made  on  this  principle  lack  one  or  the  other,  and 
sometimes  both. 

The  second  type  of  governor,  as  mentioned,  is  not  open  to  the 
objections  noted,  if  properly  designed,  inasmuch  as  it  is  a 
comparatively  easy  matter  to  make  a  balanced  hydraulic  or 
pneumatic  valve  which  can  be  worked  even  by  the  most  sensi- 
tive of  governors,  and  yet  can  apply  power  enough  to  move 
heavy  gates  as  rapidly  as  is  consistent  with  safety.  In  ad- 
dition, such  governors  can  be  made  to  work  with  a  rapidity 
depending  on  the  amount  of  change  in  speed,  so  that  if  a 
heavy  load  is  thrown  on  the  wheel,  the  relay  valve  would 
be  thrown  wide  open,  and   consequently  bring  a  great  and 


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374  ELECTRIC  TRANSMISSION  OF  POWER. 

immediate  pressure  to  bear  upon  the  gates.  In  the  so-called 
electric  governors,  the  function  of  the  governor  balls  is  merely 
to  make  in  one  direction  or  the  other  the  electrical  connec- 
tions to  a  reversible  motor  which  handles  the  gates.  This 
relay  class  of  governors  has  been  recently  worked  out  with 
considerable  care,  and  is  capable  of  giving  surprisingly  close 
regulation  even  under  widely  varying  loads,  results  compar- 
able even  with  those  obtained  from  a  steam  engine  governor. 

A  third  type  of  water-wheel  governor  is  independent  of 
any  centrifugal  device  and  operates  by  a  differential  speed 
mechanism,  so  that  wherever  the  speed  of  the  wheel  varies  from 
a  certain  fixed  speed  maintained  by  an  independent  motor, 
the  gates  are  opened  or  closed  as  occasion  demands.  The 
difficulty  here  is  to  get  a  constant  speed  which  will  not  be 
sensibly  altered  when  the  load  of  working  the  gates  is  thrown 
on  the  governor  mechanism.  Some  species  of  relay  device 
is  almost  necessary  to  the  successful  operation  of  a  differential 
governor,  but  with  such  an  adjunct  very  close  regulation 
can  be  and  is  obtained. 

Up  to  the  past  few  years  almost  all  hydraulic  governing  has 
been  by  mechanisms  of  the  first  class,  and  it  is  only  recently 
that  the  relay  idea  has  been  worked  out  carefully,  both  for 
centrifugal  and  differential  mechanisms,  so  as  to  obtain  any- 
thing like  satisfactory  results  for  electrical  work  where  close 
regulation  of  speed  over  a  wide  variation  of  load  is  very 
necessary. 

For  electrical  purposes,  several  rather  interesting  governing 
mechanisms  have  been  tried,  which  do  not  fall  into  any  of  these 
classes,  inasmuch  as  their  function  is  to  keep  the  load  con- 
stant and  prevent  variations  of  speed  instead  of  checking  these 
variations  after  they  have  been  set  up.  Such  governors  (load 
governors  they  may  properly  be  called)  operate  by  electric 
means,  throwing  into  circuit  a  heavy  rheostat  or  a  storage 
battery  when  the  electrical  load  falls  off,  and  cutting  these 
devices  out  again  when  the  load  in  the  main  circuit  increases. 
These  governors  have  in  several  instances  been  applied 
with  success  to  controlling  the  variable  loads  found  in  electric 
railway  stations  operated  by  water-wheels.  But  they  waste 
energy  in  a  very  objectionable  manner,  and  at  best  can  only 


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WATER-WHEELS.  375 

be  regarded  as  bad  makeshifts,  out  of  the  question  when  there 
must  be  any  regard  for  economy  of  water,  and  only  to  be 
tolerated  in  the  lack  of  an  efficient  speed  regulator. 

Occasionally  electric  governors  operated  by  the  variations 
in  the  voltage  of  the  circuit  supplied  have  been  tried,  but 
these  are  open  to  two  serious  objections:  In  the  first  place, 
they  do  not  hold  the  voltage  steady  for  the  same  reasons  that 
most  speed  regulators  do  not  hold  the  speed  steady.  Secondly, 
they  regulate  the  wrong  thing.  In  transmission  plants,  most 
of  which  are  and  will  be  operated  by  alternating  currents,  it 


Pig.  212. 

is  important  that  the  frequency  be  kept  uniform.  If  the  vol- 
tage is  kept  constant  by  varying  the  speed,  the  frequency  is 
subject  to  enough  variation  to  be  very  annoying  in  the  opera- 
tion of  motors.  Automatic  voltage  regulators,  working  through 
variation  of  the  field  excitation  of  the  generator,  belong  in  a 
different  category  and  have  come  into  considerable  and  suc- 
cessful use. 

To  pass  from  the  general  to  the  special,  Fig.  212  shows  a 
typical  water-wheel  governor  of  the  first  class,  that  is,  of  the 
kind  operated  directly  by  the  wheel  through  a  system  of  dogs 
worked  by  a  fly  ball  governor.  There  is  here  no  attempt  at 
delicate  relay  work,  and  the  resulting  mechanism,  while  quite 
good  enough  for  rough-and-ready  work,  is  of  little  use  for 
any  case  where  a  variable  load  must  be  held  to  its  speed  with 
even  a  fair  degree  of  accuracy.     The  cut  shows  the  construction 


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376  ELECTRIC  TRANSMISSION  OF  POWER, 

well  enough  to  render  further  description  superfluous.  Gov- 
ernors like  these  were  practically  the  best  available  for  many 
years,  and  proved  to  be  cheap  and  durable,  but  they  seldom 
governed  much  more  than  to  keep  the  wheels  from  racing 
dangerously  when  the  load  was  thrown  off,  or  from  slowing 
down  permanently  when  it  came  on.  It  is  not  too  much  to 
say  that  they  never  should  be  used  in  connection  with  an 
electrical  station,  unless  combined  with  intelligent  hand  regu- 
lation —  which  at  a  pinch  is  not  to  be  despised. 

Of  the  indirect  acting  and  relay  governors  there  are  many 
species,  most  of  which  had  better  be  consigned  to  the  oblivion 
of  the  scrap  heap.  But  out  of  the  manifold  inventions  and 
experiments  good  has  come,  so  that  at  the  present  time  there 
are  a  few  delicate  relay  governors  capable  of  holding  the  wheel 
speed  constant  within  a  very  narrow  margin  indeed.  Others 
of  similar  excellence  will  probably  be  evolved,  but  just  now 
three,  the  Lombard,  Replogle,  and  the  Faesch-Piccard,  together 
with  one  or  two  electrical  governors,  are  decidedly  the  best 
known.  The  first  named  has  given  very  remarkable  results 
in  many  transmission  plants  in  which  it  has  been  employed  — 
results  quite  comparable  with  those  obtained  from  a  well-gov- 
erned steam  engine.  The  second  has  given  excellent  results 
in  the  Oregon  City  transmission  and  elsewhere,  while  the  last 
was  adopted  for  the  original  transmission  at  Niagara  Falls 
and  has  done  its  work  well,  although  in  the  extension  of  the 
plant  an  hydraulic  relay  governor  designed  by  Escher,  Wyss 
&  Co.,  was  installed.  They  are  suitable  types  of  the  hydraulic, 
electric,  and  mechanical  relay  governors. 

The  Lombard  governor,  Plate  XIII,  is  an  hydraulic  relay  in 
principle.  The  gate-actuating  mechanism  is  a  rack  gearing 
into  a  pinion,  and  driven  to  and  fro  by  the  piston  of  a  pressure 
cylinder.  The  working  fluid  is  thin  oil,  kept  under  a  pressure 
of  about  200  lbs.  per  square  inch.  This  pressure  is  supplied 
by  a  pump  driven  by  the  pulley  shown  in  the  figure  and 
operating  to  keep  up  a  200  lb.  air  pressure  in  the  pressure 
chamber  at  the  base  of  the  governor,  above  the  oil  that  par- 
tially fills  it.  This  chamber  is  divided  into  two  sections,  the 
one  holding  the  oil  under  pressure,  the  other  being  a  vacuum 
space  kept  at  reduced  pressure  by  the  pump  system. 


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WATER-WHEELS. 


877 


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878  ELECTRIC  TRANSMISSION  OF  POWER. 

The  circulation  of  oil  is  from  the  pressure  chamber  through 
the  piping  system  and  valves  to  the  working  cylinder,  and 
thence  into  the  vacuum  chamber,  whence  it  is  pumped  back 
into  the  pressure  chamber  again.  The  governor  proper  con- 
sists of  a  sensitive  pair  of  fly  balls  operating  a  balanced  piston 
valve  in  the  path  of  the  pressure  oil.  A  motion  of  ^^  of  an 
inch  at  the  valve  is  sufficient  to  put  the  piston  into  full  action 
and  open  or  close  the  gates.  Sensitive  as  this  mechanism  is,  it 
would  not  govern  properly  without  the  addition  of  an  ingenious 
device,  peculiar  to  this  governor,  to  take  account  of  the  inertia 
of  the  system.  The  weakest  point  of  all  such  governing 
mechanisms  has  been  their  helplessness  in  the  matter  of  inertia. 
If  a  governor  even  of  the  sensitive  relay  class  be  set  to  regu- 
late a  wheel,  we  encounter  the  following  unpleasant  dilemma: 
If  the  mechanism  moves  the  gates  quite  slowly,  it  will  not  be 
able  to  follow  the  changes  of  load.  If  it  moves  them  rapidly 
the  governing  overruns  on  account  of  the  inertia  of  the  whole 
wheel  system,  so  that  the  apparatus  "hunts,"  perhaps  the 
worst  vice  a  governor  can  have  when  dynamos  are  to  be  gov- 
erned. Hence  most  governors  have  either  been  unable  to 
follow  a  quickly  varying  load  at  all,  or  they  have  made  matters 
worse  by  hunting. 

In  the  liombard  governor,  special  means  are  provided  to 
obviate  hunting.  The  bell-crank  lever  seen  in  the  background 
of  Plate  XIII  is  actuated  by  the  same  movement  that  works  the 
wheel  gates,  and  moves  the  governor  valve  independently  of 
the  fly  balls.  Its  office  is  promptly  to  close  the  valve  far  enough 
ahead  of  the  termination  of  the  regular  gate  movement  to 
compensate  for  inertia.  For  example,  if  the  speed  falls  and 
the  fly  balls  operate  to  open  the  gate  Avider,  the  lever  in  ques- 
tion closes  the  governor  valve  before  the  fly  balls  are  quite 
back  to  speed,  so  that  instead  of  overrunning  and  hunting,  the 
governing  is  practically  dead  beat. 

The  result  obtained  with  this  governor  is  well  seen  in  Fig.  213. 
This  diagram  is  taken  from  a  plant  operating  an  electric  street 
railway  —  perhaps  the  worst  possible  load  in  point  of  irregu- 
larity. The  diagram  shows  a  maximum  variation  of  2.1  per 
cent  from  normal  speed,  lasting  less  than  one  minute,  imder 
extreme  variation  of  load.     These  results  are  entirely  authen- 


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PLATE  Xin. 


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WATER-WHEELS. 


379 


tic,  the  readings  having  been  taken  jointly  by  the  represen- 
tatives of  the  governor  company  and  the  local  company.  Speed 
was  taken  by  direct  reading  tachometer  and  load  from  the 
station  instruments. 

Fig.  214  shows  a  small  governor  of  the  same  make  intended 
for  use  with  impulse  wheels,  and  for  similar  light  work  under  high 


Fiu.  214. 

heads.  It  works  on  precisely  the  same  principle  as  the  larger 
governor,  save  that  the  power  is  derived  from  a  water  cylinder 
taking  water  from  the  full  head  of  the  plant.  The  work  of 
this  little  governor  is  5.4  foot-pounds  for  each  foot  of  working 
head,  quite  enough  to  handle  the  deflecting  nozzles  or  needle 
valves  used  for  regulating  impulse  wheels.  The  larger  governor 
of  Plate  XIII  has  a  very  different  task  in  moving  the  heavy 


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880  ELECTRIC  TRANSMISSION  OF  POWER, 

gates  of  turbine  wheels  and  is  designed  to  develop  more  than 
10,000  foot-pounds.  For  still  heavier  duty  a  vertical  cylinder 
type  of  Lombard  governor,  somewhat  simplified  from  the 
forms  here  shown,  has  recently  been  introduced. 

A  gate  gauge  is  generally  attached  to  the  bed  plate  of  the 
Lombard  governor,  so  that  the  excursions  of  the  piston  plainly 
show  the  exact  extent  of  gate  opening.  The  mechanism  of 
the  governor  is  decidedly  complicated,  but  it  is  extremely 
well  made  and  fitted,  so  that  it  seldom  gets  out  of  order.  It 
permits  readily  of  all  sorts  of  adjustment  with  respect  to  the 
speed,  but  for  power  transmission  work  one  needs  constant 
speed  only,  except  when  vaiying  speed  temporarily  in  syn- 
chronizing a  generator.  The  invariable  rule,  therefore,  should 
be  to  adjust  the  governor  carefully  for  the  exact  speed  required, 
and  thereafter  to  let  its  adjustments  alone  as  long  as  it 
continues  to  hold  that  speed.  In  power  transmission  work  and 
in  railway  plants,  this  governor  is  at  present  used  probably 
more  than  all  others  combined. 

The  Faesch  &  Piccard  governor  has  taken  several  forms, 
the  idea  of  a  sensitive  relay  mechanism  being  carried  through 
all  of  them.  An  hydraulic  relay  has  been  successfully  em- 
ployed abroad.  In  this  the  function  of  the  fly  balls  is  reduced 
to  moving  a  balanced  valve  controlling  hydraulic  power 
derived  from  the  natural  head,  or  from  a  pressure  cylinder. 
There  is  no  mechanical  provision  against  hunting,  but  the 
speed  of  governing  is  adjusted  as  nearly  as  possible  to  the  re- 
quirements of  the  load,  and  the  results  are  generally  good.  In 
the  great  Niagara  plant  the  governor  is  situated  on  the  floor 
of  the  power  house,  nearly  140  feet  above  the  wheel.  It  is  a 
very  sensitive  mechanical  relay,  in  which  the  motion  of  a  pair 
of  fast  running  fly  balls  puts  into  operation  through  a  system  of 
oscillating  dogs  a  brake-tightening  mechanism,  which  in  its  turn 
permits  power  to  be  transferred  from  pulleys  driven  from  the 
turbine  shaft  through  a  pair  of  dynamometer  gears,  to  the  sjrstem 
of  gearing  that  works  the  balanced  gate  at  the  end  of  the  lever 
system  140  feet  below  the  governor.  This  governor  was  guar- 
anteed to  hold  the  speed  constant  within  2  per  cent  under  ordi- 
nary changes  of  load,  and  to  limit  the  speed  variation  to  4  per 


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ELECTRIC  TRANSMISSION  OF  POWER. 


cent  for  a  sudden  change  of  25  per  cent  in  the  load.  Fig.  215 
gives  a  good  notion  of  the  principles  of  this  apparatus,  which 
is  fairly  satisfactory.  The  Replogle  governor  is  an  electro- 
mechanical relay  shown  in  Fig.  216,  whch  exhibits  its  general 
arrangement  very  well.  The  work  done  by  the  fly  balls  is 
very  trifling  and  the  mechanism  is  both  sensitive  and  powerful. 


Fio.  216. 

Fig.  217  shows  its  performance  in  governing  a  railway  load 
under  conditions  of  unusual  severity.  As  in  Fig.  213,  20  min- 
utes of  operation  are  plotted  and  the  maximum  variation  from 
105  revolutions  per  minute,  the  normal  speed,  is  less  than  10 
revolutions,  and  that  variation  lasted  less  than  20  seconds  and 
was  due  to  the  opening  of  the  circuit-breaker.  Such  work  is 
quite  good  enough  to  meet  all  ordinary  conditions. 


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WATER-WHEELS. 


883 


gj^g£g8Sggsg§ggg|§Si8       §    i    § 


Bissisiag 


li     1 

i  liii.i  ilttl 


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ELECTRIC  TRANSMISSION  OF  POWER. 


To  a  very  different  type  of  mechanism  belongs  the  diflferen- 
tial  governor  shown  in  Fig.  218.  It  has  been  applied  widely  to 
the  governing  of  Pelton  impulse  wheels,  with  very  excellent 
results.  The  principle  involved  is  very  simple.  Two  bevel 
gears,  each  carrying  on  its  shaft  a  pulley,  are  connected  by  a 
pair  of  bevel  gears  on  a  crosswise  shaft,  forming  a  species  of 
dynamometer  gearing.  Normally  the  main  gears  are  driven 
in  opposite  directions,  the  one  at  a  constant  speed  by  a  special 


FlO.  218. 

source  of  power,  the  other  from  the  shaft  to  be  governed. 
So  long  as  the  speeds  of  these  wheels  are  exactly  equal  and 
opposite,  the  transverse  shaft  remains  stationary  in  space  and 
the  gate  moving  mechanism  attached  to  it  is  at  rest.  When, 
however,  the  working  shaft  changes  speed  under  the  influence 
of  a  change  in  load,  the  transverse  shaft  necessarily  moves  in 
one  direction  or  the  other  and  keeps  on  moving  until  the 
working  shaft  gets  back  to  speed. 

In  practice  the  main  difficulty  is  to  hold  the  constant  speed 
necessary  for  one  of  the  bevel  gears,  and  the  governor  works 
admirably  or  badly  as  this  constancy  is  or  is  not  maintained. 


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WATER-WHEELS.  385 

A  heavy  fly  wheel  on  the  constant  speed  side  is  desirable,  and 
its  motive  power  should  be  quite  independent  of  the  main 
drive.  Perhaps  the  best  result  is  obtained  by  using  a  second, 
small,  differential  governor  to  hold  the  speed  uniform  at  the 
main  governor.  With  the  high  heads  and  balanced  deflecting 
nozzles  usual  in  Pelton  wheel  practice,  this  form  of  governor 
is  very  sensitive  and  does  not  hunt  noticeably,  owing  to  the 
small  inertia  of  the  moving  parts.  It  gives  good  regulation 
under  all  conditions  except  extreme  variations  of  load  where 
the  wheel  is  loaded  beyond  the  power  of  the  jet  to  enforce 
prompt  recovery  of  speed,  and  is  well  suited  to  the  conditions 
under  which  Pelton  wheels  are  generally  used. 

The  greatest  difficulty  in  hydraulic  governing  is  that  of 
hydraulic  inertia.  Water  moves  sluggishly  through  long  and 
level  pipes,  and  its  velocity  does  not  change  promptly  enough 
for  good  governing,  unless  the  waterways  are  planned  with 
that  in  view.  If  a  wheel  is  at  the  end  of  a  long  and  gently 
sloping  penstock  it  takes  a  certain  definite  amomit  of  time  to 
get  that  water  column  under  way  or  checked  in  response  to 
the  movement  of  the  gates.  And  the  longer  this  time  con- 
stant of  the  water  colunm  the  more  difficult  it  is  to  get  accu- 
rate governing,  however  good  the  governhig  mechanism  may 
be.  For  by  the  time  the  water  gets  fairly  into  action  the  load 
conditions  may  have  changed,  and  the  governor  may  be  again 
actively  at  work  trying  to  readjust  the  speed. 

In  order  to  get  accurate  governing  it  is  absolutely  necessary 
to  keep  the  time  constant  of  the  waterways  as  small  as  pos- 
sible. To  accomplish  this  the  regulating  gate  should  obviously 
be  right  at  the  wheel  and  the  penstock  should  be  as  short  and 
as  nearly  vertical  as  possible.  The  most  favorable  condition 
for  governing  is  when  the  wheel  is  practically  in  an  open 
flume.  If  steel  penstocks  are  used  they  should  pitch  as 
sharply  down  upon  the  wheels  as  conditions  permit,  some- 
thing after  the  manner  of  Fig.  208.  If  long  head  pipes  must 
be  used  governing  will  become  difficult,  although  much  help  can 
be  obtained  from  an  open  vertical  standpipe  connected  with 
the  penstock  close  to  the  wheel.  The  contents  of  this  pipe 
serve  as  a  pressure  colunm  if  the  gate  is  suddenly  opened  and 
as  a  relief  valve  if  the  gate  suddenly  closes,  averting  the  some- 


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S86  ELECTRIC  TRANSMISSION  OF  POWER. 

times  serious  pressure  due  to  the  violently  checked  stream. 
Plate  XIV  shows  such  a  standpipe  in  action  just  after  a  heavy- 
electric  load  had  been  thrown  off.  The  water  normally  stands 
very  near  the  top  pf  the  pipe,  which  begins  to  overflow  with  a 
slight  increase  of  the  hydraulic  pressure. 

Under  high  heads  such  a  standpipe  is  of  course  impracti- 
cable, and  although  some  forms  of  relief  valve  are  of  use,  the 
conditions  of  governing  are  not  easy  until  one  comes  to  the 
impulse  wheel  with  a  deflecting  nozzle. 

Not  all  water-wheels  are  governed  with  equal  ease.  If  the 
gates  are  properly  balanced  a  comparatively  small  amoimt  of 
power  will  manage  them  promptly,  and  the  wheel  is  governed 
without  trouble.  But  there  are  some  wheels  on  the  market 
with  gates  under  so  much  unbalanced  pressure  that  proper 
governing  is  difficult  or  impossible.  There  is  no  excuse  for 
the  existence  of  such  wheels,  for  they  do  not  have  compensat- 
ing advantages,  and  they  should  be  shunned.  All  the  typical 
wheels  which  have  been  described  in  this  chapter  govern 
easily,  however,  as  do  many  others.  It  is  worth  while  to  re- 
member that  good  governing  is  absolutely  indispensable  for 
good  service,  and  although  one  finds  cases  in  which  the  load  is 
so  steady  that  the  wheels  can  almost  go  without  governing, 
such  are  rare  exceptions  to  the  general  rule. 


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PLATE  XIV. 


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CHAPTER  X. 

HYDRAULIC    DP^VELOPMENT. 

So  much  electrical  transmission  work  depends  on  the  utiliza- 
tion of  water-powers  that  it  is  worth  while  briefly  to  consider 
the  subject  of  developing  natural  falls  for  such  use.  The 
subject  is  a  large  one,  quite  enough  to  fill  a  volume  by  itself, 
and  the  most  that  can  be  done  here  is  to  point  out  the  salient 
facts  and  put  the  reader  in  possession  of  such  information  as 
will  enable  him  to  avoid  serious  blimders  and  to  take  up  the 
subject  intelligently. 

Natural  water-powers  of  course  vary  enormously  in  their 
characteristics.  In  our  own  country,  where  water-power  is 
very  widely  distributed,  we  find  three  general  classes  of  powers, 
often  running  into  each  other  but  still  sufficiently  distinct  to 
cause  the  methods  of  developing  them  to  be  quite  well  defined. 

By  far  the  best  known  class  of  powers  are  those  derived 
from  the  swift  rivers  that  are  found  in  New  England  and 
other  regions  in  which  the  general  level  of  the  country  changes 
rather  rapidly.  They  flow  through  a  country  of  rocky  and 
hilly  character,  and  large  or  small,  are  still  swift,  powerful 
streams,  with  frequent  rapids  and  now  and  then  a  cascade. 
Such  rivers  are  generally  fed  to  no  small  extent  by  springs 
and  lakes  far  up  toward  the  mountains,  and  catch  in  addition 
the  aggregated  drainage  of  the  irregular  hill  country  through 
which  they  flow.  Types  of  this  class  are  the  Merrimac  and 
the  Androscoggin  among  the  New  England  rivers,  the  upper 
Hudson,  and  many  others.  Another  and  quite  different  class 
of  powers  are  those  derived  from  the  slow  streams  that  flow 
through  a  flat  or  rolling  alluvial  country  —  the  Mississippi 
valley  and  the  lowlands  of  the  Southern  Atlantic  States.  Al- 
though possessed  of  many  tributaries  that  spring  from  among 
the  mountains,  the  great  basins  which  they  drain  form  the 
main  reliance  of  rivers  of  this  kind  —  immense  areas  of  fertile 
country  the  aggregated  rainfall  of  which  supports  the  streams. 

387 


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/         388  ELECTRIC  TRANSMISSION  OF  POWER. 

Finally,  there  are  many  fine  water-powers  that  come  from 
mountain  streams,  fed  from  little  springs  among  the  rocks, 
from  the  melting  of  the  winter  snows  and  the  drainage  of 
heights  which  the  snow  never  deserts,  and  from  the  rain 
gathered  by  desolate  gorges. 

These  moimtain  rivers  often  furnish  magnificent  powers, 
easy  and  cheap  to  develop,  but  very  variable.  In  summer  the 
stream  may  dwindle  to  a  mere  brook,  while  in  spring,  from 
the  combined  effect  of  rains  and  melting  snow,  it  may  suddenly 
increase  even  many  thousand  fold,  becoming  a  tremendous 
torrent  that  no  works  built  by  man  can  withstand.  The 
available  heads  are  often  prodigious,  from  a  few  hundred  to 
more  than  a  thousand  feet,  and  the  volume  of  water  may  seem 
at  first  sight  absurdly  small,  but  when,  as  in  the  Fresno  (Cal.) 
plant  to  be  described  later,  each  cubic  foot  flowing  per  second 
means  140  mechanical  HP  delivered  by  the  wheels,  large 
volume  is  needless. 

Upland  rivers  like  those  common  in  New  England,  seldom 
give  opportunity  for  securing  high  heads.  Most  of  the  powers 
developed  show  available  falls  ranging  from  20  to  40  feet. 
Unless  the  stream  has  considerable  volume,  such  low  heads  do 
not  yield  power  enough  to  serve  anything  but  trivial  purposes  — 
only  two  or  three  HP  per  cubic  foot  per  second.  Upland  rivers, 
however,  furnish  the  great  bulk  of  the  water-power  now  utilized, 
for  they  furnish  fairly  steady  and  cheap  power  under  favorable 
conditions.  Although  subject  to  considerable,  sometimes  formi- 
dable, freshets,  when  the  snow  is  melting  or  during  heavy  rains, 
they  are  generally  controllable  without  serious  difficulty. 

Lowland  streams  seldom  offer  anything  better  than  very 
low  heads,  rarely  more  than  10  to  15  feet,  and  consequently 
demand  an  immense  flow  to  produce  any  considerable  power. 
They  are,  however,  as  a  class  rather  reliable.  The  size  and 
character  of  the  drainage  basin  makes  extremely  low  or  ex- 
tremely high  water  rare,  and  only  to  be  caused  by  very  great 
extremes  in  the  rainfall.  Such  streams  furnish  a  vast  number 
of  very  useful  powers  of  moderate  size,  forming  a  large  aggre- 
gate but  seldom  giving  opportunity  for  any  striking  feats  of 
hydraulic  engineering,  at  least  in  our  own  coimtry,  where 
fuel  is  generally  cheap. 


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HYDRAULIC  DEVELOPMENT.  889 

In  taking  up  any  hydraulic  work  with  reference  to  electrical 
power  transmission,  or  any  other  purpose  in  fact,  the  first 
necessary  step  is  to  make  a  sort  of  reconnoissancey  to  ascertain 
the  general  topography  of  the  region,  the  available  head,  and 
the  probable  flow.  The  first  two  points  are  generally  easy  to 
determine  from  existing  surveys  or  by  a  brief  series  of  levels, 
the  last  named  requires  a  combination  of  educated  judgment 
and  careful  engineering.  The  U.  S.  Geological  Survey  maps 
are  invaluable,  when  available,  for  getting  a  preliminary  idea 
of  the  topography  and  the  probable  drainage  basin.  The  facts 
are  not  really  very  difficult  to  get  at,  but  guesswork  is  emphati- 
cally out  of  order  and  heresay  evidence  even  more  worthless 
than  usual.  The  author  has  seen  more  than  one  mighty  tor- 
rent dwindle  into  a  trout  brook  when  looked  at  through 
untinted  spectacles. 

The  only  way  to  find  out  how  much  flow  is  available  is  to 
measure  it  carefully,  if  it  has  not  already  been  measiu^d  in  a 
thorough  and  trustworthy  manner  —  not  once  or  twice  or  a 
dozen  times,  but  weekly  or,  better,  daily,  for  an  entire  season  at 
least;  the  more  thoroughly  the  better.  A  knowledge  of  the 
absolute  flow  at  one  particular  time  is  interesting,  but  of  little 
value  compared  with  a  knowledge  of  the  variations  of  flow 
from  month  to  month,  or  from  year  to  year. 

Such  a  series  of  measurements  tells  two  very  important 
things  —  first,  the  minimum  flow,  which  represents  the  max- 
imum power  available  continuously  without  artificial  storage 
of  water;  and  second,  the  aggregate  flow  dining  any  specified 
period,  which  shows  the  possibilities  of  eking  out  the  water 
supply  by  storage. 

The  methods  of  measurement  are  comparatively  simple. 
For  small  streams  the  easiest  way  is  to  construct  a  weir  across 
the  stream  and  measure  the  flow  over  a  notch  of  known  dimen- 
sions in  this  weir.  Such  a  temporary  dam  should  be  tight  and 
firmly  set,  and  high  enough  to  back  up  .the  water  into  a  quiet 
pool  free  from  noticeable  flow  except  close  to  the  edge  of  the 
weir.  There  should  be  sufficient  fall  below  the  bottom  of  the 
notch  in  the  weir  to  give  a  clear  and  free  fall  for  the  issuing 
water  —  say  two  or  three  times  the  depth  of  the  flow  over  the 
weir  itself. 


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390 


ELECTRIC  TRANSMISSION  OF  POWER. 


Fig.  219  shows  clearly  the  general  arrangement  of  a  measur- 
ing weir.  Here  A  shows  the  end  supports  of  the  weir,  here 
composed  of  a  single  plank,  while  B  is  the  lower  edge  of  the 
notch  through  which  the  water  flows.  This  edge  5,  as  well  as 
the  sides  of  the  notch,  should  be  chamfered  away  to  a  rather 
sharp  edge  on  the  upstream  side,  which  must  be  vertical. 
Back  some  feet  from  the  weir  so  as  to  be  in  still  water,  should 
be  set  firmly  a  post  E,  the  top  of  which  is  on  exactly  the  same 
level  as  the  bottom  of  the  weir  notch  B.  D  shows  this  level, 
while  the  line  C  shows  the  level  of  the  still  water.  The  quan- 
tities to  be  exactly  measured  are  the  length  of  the  notch  B 


mi^m 


¥lQ,  219. 

and  the  height  from  the  level  of  the  edge  of  5  to  the  normal 
level  surface  of  the  water  in  the  pool.  This  can  be  done 
generally  with  sufficient  accuracy  by  holding  or  fixing  a  scale 
on  the  top  of  the  post  E.  If  we  call  the  breadth  of  the  notch 
by  and  this  height  A,  both  measured  in  feet,  the  flow  in  cubic 
feet  per  minute  is 

Q^  40  cbh  yJ2gh. 

Here  g  is  32.2  and  c  is  the  "coefficient  of  contraction,"  which 
defines  the  ratio  of  the  actual  minimum  area  of  the  flowing 
jet  to  the  nominal  area  6  A. 

This  coefficient  varies  slightly  with  the  width  of  the  notch 


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HYDRAULIC  DEVELOPMENT. 


391 


as  compared  with  the  whole  width  of  the  weir  dam.  Calling 
this  w,  the  value  of  c  is  approximately 

b 
c  =  0.57  +  10  -  • 
w 

This  gives  c  =  .62  for  a  notch  half  the  width  of  the  weir  and 
c  =  .67  for  the  full  width  of  the  weir.  For  notches  below  one- 
quarter  the  width  of  the  weir  the  values  of  c  become  somewhat 
uncertain,  and  as  a  rule  6  should  be  over  half  of  w.  Further, 
the  notch  should  not  be  so  wide  as  to  reduce  the  water  flowing 
over  it  to  a  very  thin  sheet.  It  is  best  to  arrange  the  notch  so 
that  the  depth  of  water  h  may  be  anywhere  a  tenth  to  a  half  of 
b.  For  purposes  of  approximation  weir  tables  are  sometimes 
convenient.  These  give  usually  the  flow  in  cubic  feet  per 
minute  corresponding  to  each  inch  in  width  b,  for  various 
values  of  h.  Such  a  table,  condensed  from  one  used  by  one  of 
the  prominent  turbine  makers,  is  given  below.  Where  quite 
exact  measurement  is  required  the  constant  c  should  be  deter- 
mined from  the  actual  dimensions  and  a  working  table  de- 
duced from  it. 

Table  of   Wbirb. 


Incbee  and  Fractions  Depth  on  Weir. 

0 

4 

h 

1 

1 

0.40 

1.14 

2.09 

3.22 

4.61 

6.92 

7.46 

9.12 

10.88 

12.76 

14,71 

16.76 

18.89 

21.12 

23.42 

26.80 

28.26 

30.78 

0.66 

1.36 

2.36 

3.63 

4.86 

6.30 

7.87 

9.55 

11.34 

18.23 

16.21 

17.28 

19.44 

21.68 

24.01 

26.41 

28.88 

31.43 

0.74 

1.59 

2.64 

3.86 

5.25 

6.68 

8.28 

9.99 

11.80 

13.72 

16.72 

17.82 

20.00 

22.26 

24.60 

27.02 

29.51 

82.07 

0  97 

2 

1  84 

3 

2.93 

4 

4  17 

6 

5  56 

6 

7  07 

7 

8  70 

8 

10  43 

9 

12  27 

10 

14  21 

11 

16  24 

12 

18  35 

13 

20  56 

14 

22.83 

15 

25  19 

16 

27  63 

17 

30  14 

18 

82  73 

Cubic  feet  per  miDute  per  iucli  of  width. 

West  of  the  Rocky  Mountains  a  special  system  of  measuring 
water  by  ''miner's  inches"  has  come  into  very  extensive  use. 


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ELECTRIC  TRANSMISSION  OF  POWER. 


It  originated  in  the  artij&cial  distribution  of  water  for  mining 
and  irrigating  purposes,  and  has  since  extended  to  a  conven- 
tional measurement  for  streams.  The  miner's  inch  is  a  unit 
of  constant  flow,  and  varies  somewhat  from  State  to  State,  its 
amount  being  regiJated  by  statut-e  in  various  States.  It  is 
the  flow  through  an  aperture  1  inch  square  under  a  specified 
head,  frequently  6  inches.  The  method  of  measurement  is 
shown  in  Fig.  220.  The  water  is  led  into  a  measuring  box 
closed  at  the  end  except  for  an  aperture  controlled  by  a  slide. 
The  end  board  is  IJ  inch  thick,  and  the  aperture  is  2  inches 


Fio.  220. 

wide,  its  bottom  is  2  inches  above  the  bottom  of  the  box,  and  its 
centre  6  inches  below  the  level  of  the  water.  Each  inch  of 
length  of  the  aperture  then  represents  2  miner's  inches. 
Under  these  conditions  the  flow  is  1.55  cubic  feet  per  minute  for 
each  miner's  inch.  Under  a  4J  inch  effective  head,  which  is 
extensively  used  in  southern  California  and  the  adjacent 
regions,  the  miner's  inch  is  about  1.2  cubic  feet  (9  gallons) 
per  minute. 

For  streams  too  large  to  be  readily  measured  by  the  means 
already  described,  a  method  of  approximation  is  applied  as 
follows: 


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398 


Select  a  place  where  the  bed  of  the  stream  is  fairly  regular 
and  take  a  set  of  soundings  at  equal  intervals,  a,  h,  c,  d,  Fig. 
221,  perpendicular  to  the  direction  of  flow,  using  a  staff  rather 
than  a  sounding  line,  as  it  can  more  easily  be  kept  perpendicu- 
lar. Ascertain  thus  the  area  of  flow.  Then  establish  two  lines 
across  the  stream,  say  100  feet  apart  and  nearly  equidistant 
from  the  line  of  soundings.  Then  throw  floats  into  the 
stream  near  the  centre  and  time  their  passage  across  the  two 
reference  lines.  This  establishes  the  velocity  of  the  flow 
across  the  measured  cross  section.  As  the  water  at  the  bottom 
and  sides  of  the  channel  is  somewhat  retarded,  the  average 
velocity  is  generally  assumed  to  be  80  per  cent  of  that  mea- 
sured as  above  in  the  middle  of  the  stream. 

The  more  complete  the  data  on  variations  of  flow,  the 


Fia.  221. 

better.  The  most  important  point  to  be  fixed  is  the  flow  at 
extreme  low  water,  both  in  ordinary  seasons  and  seasons  of 
unusual  drought.  Except  on  very  well-known  streams  pre- 
vious data  on  this  point  are  generally  not  available.  The 
flow  should  therefore  be  measured  carefiJly  through  the  usual 
period  of  low  water  during  at  least  one  season.  From  the 
minimum  flow  thus  obtained  there  are  various  ways  of  judging 
the  miniminn  flow  in  a  very  dry  year.  Sometimes  certain 
riparian  marks  are  known  to  have  been  uncovered  in  some 
particiJar  year,  and  the  relative  flow  can  be  computed  from  the 
difference  thus  established.  Again,  the  records  of  a  series  of 
years  may  be  obtained  from  a  neighboring  stream  of  similar 
character,  and  the  ratio  between  ordinary  and  extraordinary 
minima  assumed  to  be  the  same  for  both.  This  assumption 
must  be  made  cautiously,  for  neighboring  streams  often  are 
fed  from  sources  of  very  different  stability. 


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394  ELECTRIC  TRANSMISSION  OF  POWER. 

Failing  in  these  more  direct  methods,  recourse  may  be 
taken  to  rainfall  observations.  For  this  purpose  the  rainfall 
in  the  basin  of  the  stream  shoiJd  be  measured  during  the  con- 
tinuance of  the  observations  on  flow.  By  noting  the  effect  of 
known  rainfall  on  the  flow  of  the  stream,  one  can  make  a 
fairly  close  estimate  of  the  flow  in  a  very  dry  year  in  which 
the  rainfall  is  known  by  months,  or  for  an  assumed  minimum 
rainfall.  In  a  similar  way  can  be  ascertained  the  probable 
high  water  mark,  record  of  which  is  often  left  by  debris  on 
the  banks. 

In  a  fairly  well-known  country  the  conditions  of  flow  can  be 
approximated  by  reference  to  rainfall  alone.  The  area  drained 
by  the  stream  down  to  the  point  of  utilization  can  be  closely 
estimated.  If  rainfall  observations  in  this  district  are  avail- 
able, or  can  be  closely  estimated  from  the  results  at  neighbor- 
ing stations,  one  may  proceed  as  follows:  The  total  water 
falling  into  the  basin  is  2,323,200  cubic  feet  per  square  mile 
for  each  inch  of  rainfall.  Only  a  portion  of  this  finds  its  way 
into  the  streams,  most  of  it  being  taken  up  by  seepage,  evapo- 
ration, and  so  forth.  The  proportion  reaching  the  streams 
varies  greatly,  but  is  usually  from  .3  to  .6  of  the  whole.  If  this 
proportion  is  known  from  observations  on  closely  similar 
basins  and  streams  the  total  yearly  flow  can  be  approximated, 
and  if  the  distribution  of  flow  on  a  similar  stream  is  known, 
one  can  make  a  tolerable  estimate  of  the  amount  and  condi- 
tions of  flow  in  the  stream  under  investigation. 

This  process  is  far  from  exact,  since  the  proportion  of  the 
total  water  which  is  found  in  the  streams  varies  greatly  from 
place  to  place,  and  with  the  total  rainfall  in  any  given  week  or 
day.  The  sources  of  loss  do  not  increase  with  the  total  pre- 
cipitation, and  the  only  really  safe  guide  is  regular  observa- 
tion of  the  rainfall  and  the  flow  ddring  the  same  period.  At 
times,  however,  rainfall  estimates  are  about  the  only  source 
of  information  available  and  when  made  with  judgment  are 
decidedly  valuable.  In  a  well  investigated  country  they  are 
sometimes  surprisingly  accurate. 

A  good  idea  of  the  uncertainties  of  hydraulic  power  can  be 
gathered  from  the  recorded  facts  as  regards  the  Merrimac, 
one  of  the  most  completely  and  carefully  utilized  American 


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HYDRAULIC  DEVELOPMENT,  896 

streams,  which  has  been  under  close  observation  for  half  a 
century.  The  area  of  its  watershed  above  Lowell,  Mass.,  is 
4,093  square  miles  and  the  mean  annual  rainfall  of  the  region 
is  about  42  inches.  The  observations  of  many  years  indicate 
that  the  maximum,  minimum,  and  mean  flows  are  on  approxi- 
mately the  following  basis: 

(Spring) Maximum,  90  cubic  feet  per  minut/e  per  square  mile 

(June) Mean,  65  cubic  feet  per  minute  per  square  mile 

(August,  September) . .  Minimum,   30  cubic  feet  per  minute  per  square  mile 

The  annual  rainfall,  if  it  all  coiJd  be  reckoned  as  in  the 
stream  and  uniformly  distributed,  would  amount  to  very 
nearly  180  cubic  feet  per  minute  per  square  mile  of  watershed. 
In  fact,  this  flow  is  reached  or  passed  only  on  occasional 
days  of  heavy  freshets  during  the  spring  rains,  when  the  snow 
is  melting  rapidly.  The  normal  maximum  flow  is  just  50  per 
cent  of  the  conventional  average,  while  the  real  average  falls 
to  about  30  per  cent  and  the  minimum  to  less  than  17  per 
cent.  Of  late  years  this  minimum  has  sometimes  been  still 
smaller,  little  over  10  per  cent  instead  of  17,  a  state  of  things 
due  to  the  destruction  of  the  forests  on  the  upper  watershed. 
In  a  heavily  wooded  country  the  rainfall  is  long  retained  and 
finds  its  way  to  the  streams  slowly  and  gradually.  When 
the  forests  are  cut  off  the  water  runs  quickly  to  the  streams, 
and  the  result  is  heavy  seasons  of  freshets  when  the  snow  is 
melting  —  all  the  more  rapidly  because  of  lack  of  forest  shade 
—  and  extreme  low  water  dming  the  dry  months.  In  a  bare 
country  the  variations  of  flow  are  often  prodigious,  and  with- 
out storage  one  can  safely  reckon  only  upon  the  minimum 
flow  of  the  dryest  year.  As  the  denudation  of  the  uplands  goes 
on  hydraulic  development  will  steadily  grow  more  expensive. 

One  cubic  foot  per  second  per  square  mile  of  drainage 
area  is  a  figure  often  used  to  determine  the  average  flow  for 
which  development  should  be  planned  and  in  streams  like 
those  of  New  England  this  estimate  is  not  far  from  the  truth. 

In  some  streams,  generally  in  hot  climates,  no  small  part 
of  the  flow  is  during  the  dry  season  in  the  strata  underlying 
the  apparent  bed  of  the  stream,  and  can  be  in  part,  at  least, 
captured  by  carrying  down  the  foundations  of  the  permanent 
works. 


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896  ELECTRIC  TRANSMISSION  OF  POWER. 

When  the  flow  has  been  ascertained  the  available  HP  is 
easily  computed.  The  practicable  head  can  be  easily  deter- 
mined by  a  little  leveling.  If  H  is  this  head  in  feet  and  Q 
the  flow  in  cubic  feet  per  minute,  then  the  theoretical  HP  of 
the  stream  is 

62.4  H  Q 


HP  = 


33,000 


The  mechanical  HP  obtained  by  utilizing  the  stream  in  water- 
wheels  is  this  total  amount  multiplied  by  the  efficiency  of 
the  wheels,  usually  between  .75  and  .85.  At  80  per  cent 
efficiency  the  proceeding  formula  reduces  to 

^^-"650-' 

which  gives  the  available  mechanical  horse-power  directly.  In 
many  streams  the  available  head  is  limited  by  the  permissible 
overflow  of  the  banks  as  determined  by  the  rights  of  other 
o\vners,  or  by  danger  of  backing  up  the  stream  to  the  detri- 
ment of  powers  higher  up.  These  conditions  must  be  deter- 
mined by  a  carefiJ  survey. 

Before  taking  up  seriously  the  development  of  a  water-power 
it  is  advisable  to  enter  into  an  examination  of  the  legal  status 
of  the  matter,  which  is  sometimes  very  involved.  The  gen- 
eral principle  of  property  in  streams  is  that  the  water  belongs 
in  common  to  the  riparian  owners,  and  cannot  be  employed  by 
one  to  the  detriment  of  another.  But  each  State  has  a  set  of 
statutes  of  its  own  governing  the  use  of  water  for  power  and 
other  purposes,  often  of  a  very  complicated  character,  involved 
with  special  charters  to  storage  and  irrigation  companies  and 
other  ancient  rights,  so  that  the  real  rights  of  the  purchaser  of 
a  water  privilege  are  often  limited  in  curious  and  troublesome 
ways,  especially  when  the  stream  has  been  long  utilized  else- 
where. 

Generally  the  riparian  owners  have  fiJl  rights  to  the  nat- 
ural flow  of  the  stream,  which  is  often  by  no  means  easy  to 
determine.  The  laws  of  various  States  regarding  the  matter 
of  flowage  vary  widely,  and  altogether  the  intending  purchaser 
will  find  it  desirable  to  investigate  carefully  not  only  the  title 


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HYDRAVLtC  D^VtlLOPMENT,  397 

to  the  property,  but  the  limitations  of  the  rights  which  he 
would  acquire. 

In  streams  of  small  volume  carried  through  pipe  lines  the 
effective  head  is  diminished  by  friction  in  the  pipes.  This 
loss  has  already  been  discussed  in  Chapter  II.  In  any  case 
where  water  is  carried  in  canals  or  open  flumes  there  will  be, 
too,  a  slight  loss  of  head,  generally  trivial. 

It  often  happens  that  there  is  so  great  a  difference  between 
the  normal  flow  of  the  stream  during  most  of  the  year  and  its 
minimum  flow  during  a  few  weeks  as  to  make  it  highly  desira- 
ble to  store  water  by  impounding  it,  so  as  to  help  out  the 
sometimes  scanty  natural  supply.  With  mountain  streams 
under  high  head  this  is  frequently  quite  easy.  Even  when  it 
is  impracticable  to  impound  enough  to  help  out  during  the 
whole  low  water  period,  it  is  sometimes  very  useful  to  impound 
enough  to  last  for  a  day  or  two  in  case  of  necessary  repairs. 

A  certain  reservoir  capacity  is  quite  necessary,  so  as  to  per- 
mit the  storage  of  water  at  times  of  light  load  for  utilization 
at  times  of  heavy  load.  This  process  is  carried  out  on  a  vast 
scale  on  the  New  England  rivers,  where  the  water,  used  during 
the  day  in  textile  manufacturing,  is  stored  in  the  ponds  at 
night  as  far  as  possible.  While  electric  transmission  plants  do 
not  offer  the  same  facilities  for  storage,  since  they  generally 
run  day  and  night,  the  application  of  the  same  process  would 
often  greatly  increase  their  working  capacity  and  greatly  lower 
the  fixed  charges  per  hydraulic  HP.  Such  storage  is  espe- 
cially valuable  in  cases  where  the  water  supply  is  limited,  as  it 
often  is  in  plants  working  under  high  heads.  Every  cubic  foot 
of  water  is  then  valuable  and  should  be  saved  whenever  pos- 
sible. Regulation  by  deflecting  nozzles,  which  is  very  generally 
employed  in  this  class  of  plants,  is  particularly  objection- 
able on  the  score  of  economy,  and  ought  to  be  replaced  by 
some  more  efficient  method. 

As  an  example  of  what  can  be  done  with  storage  under  high 
heads,  it  happens  that  at  650  feet  effective  head  oae  mechani- 
cal HP  requires  almost  exactly  one  cubic  foot  of  water  per 
minute  at  80  per  cent  wheel  efficiency.  For  a  500  HP  plant, 
then,  the  water  required  is  30,000  cubic  feet  per  hour. 

One  can  store  43,560  cubic  feet  per  acre  per  foot  of  depth, 


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398  ELECTRIC  TRANSMISSION  OF  POWER. 

so  that  a  single  acre  10  feet  deep  would  store  water  enough  to 
operate  the  plant  at  full  load  for  14^  hours,  or  under  ordinary 
conditions  of  load  for  a  full  day.  If  the  flow  in  the  stream 
were  only  15,000  cubic  feet  per  hour  in  time  of  drought,  the 
acre  would  yield  two  days  supply  and  15  acres  would  carry  the 
plant  for  a  month.  Such  storage  is  common  enough  in  irriga- 
tion work,  and  is  capable  of  enormously  increasing  the  work- 
ing capacity  of  a  transmission  plant,  even  at  a  head  much  less 
than  that  mentioned. 

With  even  100  feet  available  head,  it  is  comparatively  easy  to 
impound  water  enough  to  assist  very  materially  in  tiding  over 
times  of  heavy  load  and  in  increasing  the  available  capacity. 


Fig.  22,2. 

A  survey  with  storage  capacity  in  view  should  be  made  when- 
ever storage  is  possible,  and  the  approximate  cost  of  storage 
determined.  A  little  calculation  will  show  in  how  far  it  can 
be  made  to  pay. 

In  general  the  utilization  of  a  water-power  consists  in  lead- 
ing the  whole  or  a  part  of  a  stream  into  an  artificial  channel, 
conducting  it  in  this  channel  to  a  convenient  point  of  utiliza- 
tion, and  then  dropping  it  back  through  the  water-wheels 
into  the  channel  again,  usually  via  a  tail-race  of  greater  or  less 
length. 

Except  where  there  is  a  very  rapid  natural  fall  a  sub- 
stantial dam  is  necessary,  which  backs  up  the  water  into  a 
pond,  usually  gaining  thus  a  certain  amount  of  head,  whence 
the  water  is  led  in  an  open  canal  to  some  favorable  spot  from 
which  it  can  be  dropped  back  into  the  channel  at  a  lower  level. 


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HYDRAULIC  DEVELOPMENT, 


399 


The  canal  may  vary  in  length  from  a  few  rods  to  several  miles, 
according  to  the  topography  of  the  coimtry.  The  tail-race  lead- 
ing the  water  from  the  wheels  back  to  the  stream  is  short,  except 
in  rare  instances  like  the  great  Niagara  plant.  In  this  case, 
shown  somewhat  roughly  in  Fig.  222,  the  usual  construction 
was  reversed.  To  obtain  ample  clear  space  for  manufacturing 
sites  and  the  like,  the  water  was  utilized  by  constructing  above 
the  cataract  an  artificial  fall  at  the  bottom  of  which  the  wheels 
were  placed.  From  the  bottom  of  this  huge  shaft,  cut  178  feet 
deep  into  the  solid  rock,  the  water  is  taken  back  into  the 


Fig.  223. 

river  through  a  tunnel  7,000  feet  long,  which  constitutes  the 
tail-race. 

In  the  case  of  mountain  streams  having  a  very  rapid  fall, 
the  dam  is  often  quite  insignificant,  serving  merely  to  back  up 
the  water  into  a  pool  from  which  it  may  be  conveniently  drawn, 
and  in  which  the  water  may  be  freed  of  any  sand  that  it  carries, 
or  even  to  deflect  a  portion  of  the  water  for  the  same  purpose. 
In  such  cases  the  water  is  usually  carried  in  an  iron  or  steel 
pipe,  following  any  convenient  grade  to  the  bottom  of  the  fall 
chosen,  at  which  point  its  full  pressure  becomes  available. 

In  ordinary  practice  at  moderate  heads  the  volume  of  water 
has  to  be  so  considerable  for  any  large  power  as  to  make  a 
long  canal  very  expensive.  Further,  it  usually  happens  that 
the  topography  of  the  coimtry  is  such  as  to  make  it  very  diffi- 
cult to  gain  much  head  by  extending  the  canal.  Thus  the 
points  chosen  for  power  development  must  be  those  where 
there  is  a  rather  rapid  descent  for  a  short  distance  —  falls  or 
considerable  rapids.    Then  a  dam  of  moderate  height  gives  a 


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400  ELECTRIC  TRANSMISSION  OF  POWER. 

fair  head  by  simply  carrying  the  canal  to  a  point  where  the 
water  can  be  readily  returned  to  the  stream  below  the  natural 
fall.  The  more  considerable  this  fall  the  less  need  for  an 
elaborate  dam,  which  may  become  simply  a  means  of  regulat- 
ing the  flow  of  water  without  noticeably  raising  the  head. 

A  fine  example  of  this  sort  of  practice  is  shown  in  Fig.  223, 
which  shows  a  plan  of  the  hydraulic  development  of  the  falls 
of  the  Willamette  River  at  Oregon  City,  Ore.  The  river  at 
this  point  gives  an  estimated  available  HP  of  50,000  under  40 
feet  head.  The  stream  plimges  downward  over  a  precipitous 
slope  of  rough  basalt,  and  the  low  dam  which  follows  the  some- 
what irregular  shape  of  the  natural  fall,  is  hardly  more  than  an 
artificial  crest  to  guide  the  water  toward  the  canal  on  the  west 


Fia.  224. 

bank  of  the  stream.  This  canal  has  recently  been  widened, 
and  both  constructions  are  shown  in  the  figure.  The  fine  three- 
phase  transmission  plant  of  the  Portland  General  Electric  Com- 
pany now  faces  on  the  new  canal  wall  near  the  section  G.  At 
the  end  of  the  canal  downstream  a  series  of  locks  lead  down  to 
the  lower  river,  making  the  falls  passable  for  river  craft.  Only 
a  small  part  of  the  available  power  is  as  yet  used. 

Almost  ever}'^  river  presents  peculiarities  of  its  own  to  the 
hydraulic  engineer.  Generally  the  dam  is  a  far  more  promi- 
nent part  of  the  work  than  at  Oregon  City,  and  adds  very 
materially  to  the  head.  Choosing  a  proper  site  for  the  dam, 
and  erecting  a  suitable  structure,  requires  the  best  skill  of  the 
hydraulic  engineer.  Bearing  in  mind  that  the  function  of  a 
dam  is  to  merely  retain  and  back  up  the  flowing  water,  it  is 
evident  that  it  may  be  composed  of  a  vast  variety  of  materials 


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HYDRAULIC  DEVELOPMENT. 


401 


put  together  in  all  sorts  of  ways.     Stone,  logs,  steel,  all  come 
into  play  combined  with  each  other  and  with  earth. 

The  character  of  the  river  bed  which  furnishes  the  founda- 
tion is  a  very  important  factor  in  determining  the  material  and 
shape  of  the  dam  used.  When  the  bed  is  of  rock  or  of  that  hard 
packed  rubble  which  is  nearly  as  solid,  a  well-built  stone  dam 
is  the  best,  as  it  is  also  the  costliast,  construction.  For  such 
work  the  way  is  cleared  by  a  coffer  dam  and  the  masonry  is 


TOP  OF  EHUTTES  AND 
^TOP  OF  OAy  IK  IHUTTER 

OPEhthi::,&MuTTEFi  r 

j    BY  Q  HVDRAUUC  HAMS 


SKCTION  OP  DAM 
(thrust — IQTI  TOKS.        \ 
1  STABILITY— 7070  TONS,  f 

Containing  37,000  cu.  yaras  of  masonry. 
Fig.  226. 

laid,  if  possible,  directly  upon  the  bedrock.  When  the  bottom 
is  hard  pan  a  deep  foundation  for  the  masonry  is  almost  as 
good  as  the  ledge  itself,  while  on  a  gravel  bottom  sheet  piling 
is  sometimes  driven  and  the  stone  work  built  aroimd  it.  The 
ground  plan  is  very  frequently  convex  upstream,  giving  the 
effect  of  an  arch  in  resisting  the  pressure  of  the  water.  Fig. 
224  shows  a  section  of  a  typical  masonry  dam,  built  over  sheet 
piling  in  heavy  gravel.  This  particular  dam  is  22  feet  6  inches 
high  and  nearly  300  yards  long.  The  coping  is  of  solid  granite 
slabs  a  foot  thick.     Below  the  dam  lies  the  usual  apron  of 


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402  ELECTRIC  TRANSMISSION  OF  POWER, 

timber  and  concrete,  with  timber  sills  anchored  into  the  dam 
itself.  The  flooring  of  the  apron,  of  12  x  12  inch  timbers  laid 
side  by  side,  is  bolted  to  the  fomidation  timbers  laid  in  the 
concrete.  The  purpose  of  this  apron,  as  of  such  structures  in 
general,  is  to  prevent  imdermining  of  the  dam  by  the  eddies 
below  the  fall. 

A  still  finer  example  of  the  masonry  dam  is  shown  in  Fig. 
225  —  the  great  dam  of  the  Folsom  Water  Power  Company 
across  the  American  River  at  Folsom,  Cal.  It  is  built  of 
hewn  granite  quarried  on  the  spot,  and  is  founded  on  the  same 
ledge  from  which  the  material  was  taken.  The  abutments 
likewise  are  built  into  the  same  ledge.  On  the  crest  of  the 
dam  proper  is  a  huge  shutter  or  flash  board,  185  feet  long, 
capable  of  being  swiuig  upward  into  place  by  hydraulic  power. 
When  thus  raised  it  gives  an  added  storage  capacity  of  over 
13,000,000  cubic  yards  of  water  in  the  basin  above.  This 
dam  furnishes  power  for  the  Folsom-Sacramento  transmission, 
now  part  of  the  immense  network  of  the  California  Gas  and 
Electric  Co.,  and  it  ranks  as  one  of  the  finest  examples  of 
hydraulic  engineering  in  existence.  Including  the  abutments 
it  is  470  feet  long,  and  the  crest  of  the  abutments  towers 
nearly  100  feet  above  the  foundation  stones.  Its  magnificent 
solidity  is  not  extravagance,  for  the  American  River  carries 
during  the  rainy  season  an  enormous  volume  of  water,  filling 
the  channel  far  over  the  crest  of  the  dam  when  at  its  maximum 
flow.  There  are  few  streams  where  greater  strains  would  be 
met. 

While  these  masonry  dams  are  splendidly  strong  and  endur- 
ing, they  are  also  very  expensive,  and  hence  unless  actually 
demanded  for  some  great  permanent  work  are  less  used  than 
cheaper  forms  of  construction.  In  many  situations  these  are 
not  only  cheaper  in  first  cost,  but  even  including  deprecia- 
tion. There  are  divers  forms  of  timber  dam  which  have  given 
good  service  for  many  years  at  comparatively  small  expense. 
Of  such  dams,  timber  cribs  ballasted  with  stone  are  probably 
under  average  conditions  the  best  substitute  for  solid  masonry. 
These  crib  dams  when  well  built  of  good  materials,  are  very 
durable  and  need  few  and  infrequent  repairs.  Some  such 
dams,  replaced  after  twenty-five  or  thirty  years  in  the  course  of 


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HYDRAULIC  DEVELOPMENT. 


408 


tKry  LimAjt    I 


changing  the  general  hydraulic  conditions,  have  shown  timbers 
as  solid  as  the  day  they  were  put  down,  and  capable  of  many 
years'  further  service. 


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404  ELECTRIC  TRANSMISSION  OF  POWER. 

A  fine  example  of  such  construction  is  the  dam  of  the  Con- 
cord (N.  H.)  Land  &  Water  Power  Company,  at  Sewall's 
Falls  on  the  Merrimac.  A  section  of  this  structure  is  shown 
in  Fig.  226.  The  foundation  is  in  the  main  gravel,  in  which  the 
dam  is  made  secure  by  sheet  piling  and  stone  ballast.  The 
structure  is  essentially  a  very  solid  timber  crib  with  a  very  long 
apron.  The  total  head  is  23  feet,  of  which  more  than  half  is 
due  to  the  dam,  as  shown  in  the  levels.  The  apron  is  armored 
with  five-sixteenths  inch  steel  plate,  the  better  to  withstand  the 
bombardment  of  stray  logs  to  which  it  is  sometimes  subjected. 
The  abutments  are  of  granite.  It  has  proved  very  serviceable, 
having  successfully  withstood  several  tremendous  freshets  with 
no  damage  save  some  undermining  of  one  of  the  abutments, 


-  •-  }i  Bods 

I  Hkh  W>wr  I^TCl 

CnMoTfiKm 


^ 

fe^* 

IX  lUck  a  ft  OmttM 

iiMMfntkDiiii 

toterSafe 

rmg;^f!sm0^'^^ 

^iilSK^Xf4eikii^^ 

1*                ^ 

1 

Fia.  237. 

which  has  been  repaired  with  crib  work.  Considering  the 
character  of  the  river  bed,  this  dam  is  probably  as  reliable  as 
one  of  masonry,  and  its  cost  was  little  over  half  that  of  a 
masonry  dam. 

For  small  streams  these  ballasted  timber  dams  are  admir- 
able, and  Uttle  more  is  needed  in  most  cases. 

Another  very  convenient  and  useful  form  of  dam,  of  which 
many  examples  have  of  late  been,  erected,  is  shown  in  Fig.  227. 
It  is  a  concrete  and  steel  dam  of  the  gravity  type  in  which  the 
dam  is  given  stability  far  above  that  due  to  its  structural 
weight,  by  the  weight  of  the  superincumbent  water.  It  is 
unusual  in  that  it  really  follows  modem  architectural  lines 
instead  of  conventional  hydraulic  construction,  and  the  form 
here  shown,  devised  by  Mr.  Ambursen,  of  Watertown,  N.  Y., 
involves  a  good  many  novel  features.    Fig.  228  gives  a  section 


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HYDRAULIC  DEVELOPMENT.  405 

of  the  dam  which  is  essentially  an  inclined  concrete  and  steel 
floor  supported  by  concrete  buttresses.  The  dam  here  shown 
is  about  12  feet  high  and  the  supporting  buttresses  are  of  the 
section  indicated  in  the  figure,  12  inches  thick  and  spaced  6 
feet  between  centres.  Along  the  upstream  slope  of  these 
buttresses  there  are  set  in  the  concrete,  connecting  the  buttresses, 
a  series  of  f  inch  twisted  steel  rods  about  8  inches  between 
centres,  and  just  below  them  covering  the  whole  slope  are 
sheets  of  heavy  "expanded  metal."  Over  and  about  this  steel 
substructure  is  laid  a  tight  concrete  floor  about  6  inches  thick, 
merging  at  the  toe  of  the  dam  into  a  massive  concrete  shoe 
filling  the  space  between  the  buttresses  and  built  upon  the 
foundation  ledge.  Near  the  top  of  the  dam  the  slope  is  made 
with  a  hard  finish  of  rich  concrete  and  the  top  itself  is  made 
extra  heavy  to  resist  the  rush  of  the  water. 

These  dams  are  sometimes  built  with  concrete  downstream 
faces  and  aprons,  and  in  fact  may  take  any  form  that  occasion 
requires.  They  are  tight  and  strong,  and  ought  to  prove 
durable,  while  the  cost  is  usually  little  more  than  that  of  a 
timber  crib.  They  are,  like  concrete  work  generally,  quickly 
erected,  and  seem  specially  adapted  to  long  runs  of  moderate 
height,  although  they  are  being  used  for  heights  of  30  feet 
and  more,  and  when  properly  designed  would  appear  to  be  as 
generally  applicable  as  any  other  construction.  A  similar 
construction  can  sometimes  be  advantageously  used  for  flumes 
and  canal  walls,  since  concrete  work  can  be  done  with  material 
easily  available,  and  with  a  very  small  proportion  of  skilled 
labor,  and  when  well  done  is  both  strong  and  durable. 

The  type  of  dam  selected  for  any  particular  case  is  governed 
by  the  hydraulic  requirements  and  the  conditions  at  the 
proposed  site,  and  the  relative  costs  can  only  be  settled  by 
close  estimates.  Sometimes  massive  rubble  masonry  is  about 
as  cheap  as  anything  else,  while  in  other  circumstances  con- 
crete or  timber  would  show  the  minimum  cost. 

The  canals  leading  the  wat^r  to  the  wheels  are  of  construc- 
tion as  varied  as  the  dams,  depending  largely  on  the  nature  of 
the  ground.  Sometimes  they  are  merel}''  earthwork,  oftener 
they  are  lined  with  timber,  concrete,  or  masonry.  Canal  con- 
struction is  a  matter  to  be  decided  on  its  merits  by  the 


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406 


ELECTRIC  TRANSMISSION  OF  POWER, 


hydraiJic  engineer,  and  very  little  general  advice  can  be 
given.  For  low  heads  wooden  pipes  made  of  staves  like  a  bar- 
rel and  hooped  with  iron  every  3  or  4  feet  are  sometimes  used. 
In  many  situations  this  construction  is  cheaper  than  steel  pipe 
and  answers  admirably.  Such  wooden  pipes  are  considerably 
employed  in  the  West,  the  material  being  generally  redwood, 
and  have  proved  remarkably  durable,  some  having  been  in  use 


Fio.  228. 

for  more  than  twenty  years.     Open  timber  flumes  are  also 
widely  used. 

For  very  high  heads,  canals  and  flumes  are  almost  univer- 
sally replaced  by  iron  or  steel  riveted  pipe  taken  by  the  nearest 
rout<j  to  the  wheels  below.  This  practice  has  been  general 
on  the  Pacific  coast  and  has  given  admirable  resiJts.  The 
pipes  are  asphalted  inside  and  out  to  prevent  corrosion,  and 
some  pipe  lines  have  been  in  service  for  a  quarter  of  a  cen- 
tury without  marked  deterioration.  Large  pipes  and  those  for 
very  heavy  pressures  are  usually  made  of  mild  steel.     The 


pipes  are  castomarily  made  in  sections  for  shipment,  from 
20  to  30  feet  long,  and  the  slip  joints  are  riveted  or  packed  on 
the  ground.  For  transportation  over  very  rough  country  and 
for  very  large  pipes,  the  sections  may  be  no  more  than  2  or 
3  feet  long.  The  joints  are  then  asphalted  on  the  ground. 
Fig.  228  shows  several  of  these  short  sections  joined  together, 
exhibiting  the  nature  of  the  riveting  and  the  terminal  slio 
joint. 


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HYDRAULIC  DEVELOPMENT, 


407 


In  running  such  a  pipe  line  it  is  usually  taken  in  as  straight 
a  course  as  possible,  and  is  laid  over,  on,  or  under  the  ground  as 
occasion  requires,  usually  on  the  surface,  conforming  to  its  gen- 
eral contour.  In  long  lines  the  upper  end  is  somewhat  larger 
and  thinner  than  the  lower,  which  has  to  withstand  the  heavy 
pressure.  Fig.  229,  which  is  a  profile  to  scale  of  the  pipe  line 
of  the  noted  San  Antonio  Canon  plant  in  southern  California, 
gives  an  excellent  idea  of  good  modem  practice  in  this  sort  of 
work.  There  is  here  a  total  fall  of  about  400  feet  in  a  distance 
of  2,000  feet.  The  main  pipe  is  30  inches  in  diameter,  and  the 
steel  is  of  the  gauges  indicated  on  the  various  sections.    At 


p»Jt*itii«uiD^  Of  IM4  ua  illp  julfiti , 


FlO.  230. 

the  crests  of  two  undulations,  air  valves  are  placed  to  ensure 
a  solid  and  continuous  column  of  water  in  the  pipe.  The  last 
540  feet  of  pipe  is  reduced  to  24  inches  and  the  gauge  of  steel 
is  somewhat  heavier.  The  total  length  of  the  pipe  line  is  2,370 
feet.  To  protect  the  pipe  against  great  changes  of  tempera- 
ture it  was  loosely  covered  with  earth,  rock,  and  brush  when- 
ever possible.  At  two  sharp  declivities  the  pipe  was  anchored 
to  the  rock. 

The  general  method  of  anchoring  on  a  steep  incline  is  shown 
in  Fig.  230.  In  this  case  the  slip  joint  is  simply  calked,  and 
where  consecutive  sections  are  at  an  angle,  a  short  sleeve  is 
fitted  over  the  joint  and  lead  is  run  in  as  shown  in  the  cut. 


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408 


ELECTRIC  TRANSMISSION  OF  POWER. 


Often  a  packed  slip  joint  is  used  very  freely,  thereby  gaining 
in  flexibility,  and  riveted  joints  may  be  only  used  occasionally. 
The  line  is  generally  started  from  the  lower  end  and  the  joints 
or  the  whole  interiors  of  the  sections  asphalted  as  they  are  laid. 
The  following  table  gives  the  properties  of  steel  hydraulic 
pipe  of  the  sizes  in  common  use,  and  double  riveted: 


Diameter  in  inches 

10 

12 

14 

16 

18 

20 

84 

30 

36 

42 

Area  in  square  inches. 

78 

113 

153 

201 

254 

314 

452 

706 

1,017 

1,385 

Cabic  feet  per  minute  at 
three  foot  per  second 

100 

142 

200 

255 

320 

400 

570 

890 

1,300 

1,760 

Weight  in  pounds  per 
foot 

19.25 

22.76 

26 

29.6 

34 

36.5 

43.6 

54 

67 

74.6 

Safe  head  in  feet 

900 

750 

650 

560 

600 

460 

375 

300 

150 

135 

Change  in  safe  head  for 
each  gauge  number. . 

100 

90 

80 

70 

00 

55 

45 

35 

20 

20 

The  pipe  is  assumed  to  be  of  No.  10  gauge  steel,  and  the 
changes  in  safe  head  are  of  course  approximate  only,  but  hold 
with  sufficient  exactness  for  a  variation  of  four  or  five  gauge 
numbers.  It  is  better  to  use  a  pipe  too  thick  than  one  too 
thin,  and  to  use  extra  heavy  pipe  at  bends.  Where  the  ground 
permits,  the  water  can  often  be  carried  to  advantage  in  a  flume 
or  ditch,  and  then  dropped  through  a  comparatively  short  pipe 
line.  For  heads  approaching  or  surpassing  1,000  feet  it  is  prob- 
ably safer  to  use  lap-welded  tube  for  the  lower  portion  of  the 
run.  In  every  case  suspended  sand  must  be  kept  out  of  the 
water,  else  it  will  cut  the  wheels  and  nozzles  Ijke  a  sand  blast. 
When  one  remembers  that  under  400  feet  head  the  spouting 
velocity  of  the  water  is  about  160  feet  per  second,  the  need  of 
this  precaution  is  evident.  A  large  settling  tank  is  usually 
provided  at  the  head  works,  spacious  and  deep  enough  to  let 
the  pipe  draw  from  the  clear  surface  water.  At  its  lower  end 
the  pipe  line  terminates  in  a  receiver  —  a  heavy  cylindrical 
steel  tank  of  considerably  larger  diameter  than  the  pipe  prop)er, 
from  which  water  is  distributed  to  the  wheels. 

On  very  high  heads  a  relief  valve  is  attached  at  or  near  the 
receiver  to  avert  danger  from  a  sudden  increase  in  pressure  in 
the  pipe,  such  as  might  be  caused  by  some  sudden  obstruction 
at  the  gate. 


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HYDRAULIC  DEVELOPMENT,  409 

This  pipe  line  method  of  supply  is  considerably  used  for 
turbines  of  moderate  size  on  heads  as  low  as  75  to  100  feet,  in 
cases  where  the  natural  fall  of  the  stream  is  rather  sudden.  It 
really  amounts  to  a  considerable  elongation  of  the  iron  pen- 
stock which  is  in  common  use.  Whenever  there  is  a  sharp 
declivity  in  difficult  country,  piping  is  often  easier  and  cheaper 
than  constructing  a  sinuous  flume  or  canal.  In  such  situa- 
tions the  pipes  may  be  5  or  6  feet  in  diameter  or  even  more, 
and  being  under  very  moderate  pressure,  may  be  comparatively 
light  and  cheap. 

In  cold  climates  ice  is  one  of  the  difficulties  most  to  be 
dreaded  in  hydraulic  work.  In  high-pressure  pipe  lines  there 
is  little  to  fear,  for  fast-running  water  does  not  freeze  easily 
and  the  pipes  can  generally  be  readily  covered,  as  in  the  San 
Antonio  Canon  plant,  enough  to  prevent  freezing.  Large 
canals  simply  freeze  over  and  the  interior  water  is  thus  pro- 
tected. But  in  cold  climates  there  is  considerable  danger  of 
the  so-called  anchor-ice.  This  is,  in  extremely  cold  weather, 
formed  on  the  bed  and  banks  of  rapid  and  shallow  streams. 
The  surface  does  not  freeze,  but  the  water  is  continually  on 
the  point  of  freezing  and  flows  surcharged  with  fine  fragments 
of  ice  that  pack  and  freeze  into  a  solid  mass  with  the  freezing 
water  rapidly  solidifying  about  it.  When  in  this  condition  it 
rapidly  clogs  the  racks  that  protect  the  penstocks,  and  even 
the  wheel  passages  themselves.  In  extremely  cold  climates 
xmder  similar  circumstances  the  water  becomes  charged  with 
spicular  ice  crystals  known  as  frazil  in  Canada,  far  worse  to 
contend  with  than  ordinary  anchor-ice. 

The  best  protection  against  ice  is  a  deep,  quiet  pond 
above  the  dam,  in  which  no  anchor-ice  can  form,  and  which 
will  attach  to  its  own  icy  covering  any  fragments  that  drift 
down  from  above.  In  case  of  trouble  from  anchor-ice,  about 
the  only  thing  to  do  is  to  keep  men  working  at  the  racks  with 
long  rakes,  preserving  a  clear  passage  for  the  water.  If  the 
wheel  passages  begin  to  clog  there  is  no  effective  remedy. 

The  most  dangerous  foe  of  hydraulic  work  is  flood.  The 
precautions  that  can  be  taken  are,  first,  to  have  the  dam  and 
head-works  very  soKd,  and  second,  so  to  locate  them  if  possible 
as  to  have  an  adequate  spillway  over  which  even  a  very  large 


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410  ELECTRIC  TRANSMISSION  OF  POWER. 

amount  of  surplus  water  can  flow  without  endangering  the 
main  works.  If  a  pipe  line  is  used  it  must  be  laid  above  high 
water  mark,  else  the  first  freshet  will  probably  carry  it  away. 
The  power  station  must  likewise  be  out  of  reach  even  of  the 
highest  water. 

Closely  connected  with  the  subject  of  floods  is  that  of  varia- 
ble head,  which  in  many  streams  is  a  constant  source  of  diffi- 
culty. In  times  of  flood  the  extra  height  of  the  water  above 
the  dam  is  generally  useless,  while  the  tail-water  rises  and 
backs  up  into  the  wheels,  cutting  down  their  power  and  speed, 
often  very  seriously.  This  matter  has  already  been  discussed 
in  Chapter  IX,  in  so  far  as  it  is  connected  wdth  the  arrange- 
ment of  the  turbines.  At  very  high  heads  this  trouble  van- 
ishes, as  no  possible  variation  of  the  water  level  can  be  a 
considerable  fraction  of  the  total  head. 

The  most  delicate  questions  involved  in  hydraulic  develop- 
ment are  those  connected  with  variable  water  supply.  Having 
ascertained  as  nearly  as  possible  the  minimum  flow,  the  mini- 
mum natural  continuous  supply  of  power  is  fixed,  but  it  remains 
to  be  determined  how  the  water  in  excess  of  this  shall  be 
utilized,  if  at  all. 

Three  courses  are  open  for  increasing  the  available  mer- 
chantable power.  First,  water  can  be  stored  to  tide  over  the 
times  of  small  natural  supply.  Second,  a  plant  can  be  installed 
to  utilize  what  water  is  available  for  most  of  the  year  and  can 
be  curtailed  in  its  operation  during  the  season  of  low  water. 
Third,  the  service  can  be  made  continuous  by  an  auxiliary 
steam  plant  in  the  power  station.  Storage  of  water  can 
obviously  be  used  in  connection  with  either  of  the  other 
methods. 

Under  very  high  heads  storage  is  always  worth  undertaking 
if  the  lay  of  the  land  is  favorable.  This  of  course  means  a 
dam,  but  not  necessarily  a  very  high  or  costly  (me.  If  possible 
the  storage  reservoir  should  be  a  little  off  the  main  flow  of  the 
stream  so  as  to  escape  damage  from  freshets.  Reverting  to 
our  previous  example  of  storage,  suppose  we  have  500  HP 
available  easily  for  nine  months  of  the  year,  but  a  strong 
probability  of  not  over  250  HP  for  the  remaining  three  months. 
We  have   already  seen   that   under  these   circumstances   15 


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HYDRAULIC  DEVELOPMENT.  411 

acres  flooded  10  feet  deep  wUl  keep  up  the  full  supply  for  a 
month.  If  say  50  acres  can  be  thus  flooded,  the  all-the-year- 
round  capacity  of  the  plant  will  be  doubled.  In  the  moun- 
tainous localities  where  such  heads  are  to  be  found,  land  has 
usually  only  a  nominal  value,  and  impounding  the  equivalent 
of  this  amoimt  of  water  is  frequently  practicable.  If  it  can  be 
done  at  say  a  cost  of  $75,000,  the  annual  charge  per  HP  stored, 
coimting  interest  and  sinking  fund  at  8  per  cent,  will  be  $24, 
and  the  investment  would  generally  be  a  profitable  one.  If 
the  storage  cost  $100,000,  the  annual  charge  would  be  $32, 
and  this  would  not  infrequently  be  well  worth  the  while,  when 
power  could  be  sold  for  a  good  price. 

At  lower  heads  the  annual  charge  per  HP  stored  would  be 
considerably  greater  for  the  same  total  expenditure.  Some- 
times, however,  storage  capacity  can  be  much  more  cheaply 
gained  for  both  high  and  low  heads,  at  for  instance  not  more 
than  half  the  charge  just  mentioned.  The  matter  is  always 
worth  investigating  thoroughly  when  there  is  doubt  about 
supplying  the  power  market  with  the  natural  flow.  The  points 
to  be  looked  into  are  the  nature  and  extent  of  the  low  water 
period,  and  the  cost  of  developing  various  amounts  of  storage 
capacity.  Sometimes  the  period  of  extreme  low  water  is 
much  shorter  than  that  assumed,  and  storage  is  correspond- 
ingly cheaper. 

There  are  some  cases  in  which  it  is  possible  to  supply  cus- 
tomers with  power  for  nine  or  ten  months  in  the  year,  falling 
back  on  the  individual  steam  plants  in  the  interim.  When 
transmitted  power  can  be  cheaply  had,  it  is  worth  while  for 
the  power  user  who  is  paying  say  $100  per  HP  per  year  for 
steam  power,  to  take  electric  power  at  $50  per  HP  per  year  for 
nine  months,  and  to  use  steam  the  other  three  months.  Certain 
industries,  too,  are  likely  to  be  comparatively  inactive  in  mid- 
summer, or  may  find  it  worth  while  to  force  their  output 
during  the  months  when  cheap  power  is  obtainable,  and  shut 
down  or  run  at  reduced  capacity  when  the  power  is  unavail- 
able. This  is  a  matter  very  dependent  on  local  conditions, 
and  while  the  demand  for  such  partial  power  supply  is  gener- 
ally limited,  there  are  many  cases  in  which  it  would  be  advan- 
tageous  for   all    parties    concerned.     In    some    mountainous 


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412  ELECTRIC  TRANSMISSION  OF  POWER. 

regions,  winter  is  the  season  of  low  water  owing  to  freezing, 
and  vanous  industries  are  suspended  which  may  be  profitably 
supplied  with  power  when  the  winter  unlocks  its  gates. 

Eking  out  the  water  supply  by  an  auxiliary  steam  power 
station  is  likewise  not  of  general  appUcabiUty,  but  sometimes 
may  prove  advantageous.  It  is  most  likely  to  prove  useful  in 
locaUties  where  a  steam  power  plant  would  pay  by  virtue  of 
the  economy  due  to  production  on  a  large  scale  and  distribu- 
tion to  small  users.  Cheap  water-power  a  large  part  of  the 
year  then  abundantly  justifies  adjunct  steam-power  when 
necessary.  The  moral  effect  of  continuous  power  supply 
is  valuable  in  securing  a  market.  Whether  such  a  supply  is 
profitable  depends  on  the  ratio  between  the  cost  of  water- 
power  and  the  cost  of  steam-power.  And  it  must  not  be  for- 
gotten that  steam-power  for  two  or  three  months  in  the  year  is 
relatively  much  more  costly  than  continuous  power. 

The  general  charges  are  the  same,  although  labor,  coal,  and 
miscellaneous  supplies  decrease  nearly  as  the  period  of  opera- 
tion. Consequently,  since  there  is  this  large  fixed  item, 
amounting  to  from  20  to  40  per  cent  of  the  total  annual  cost, 
the  cost  of  power  in  a  plant  operated  only  three  months  will  be 
relatively  at  least  50  per  cent  greater  than  if  it  were  in  con- 
stant operation.  There  must  be  a  large  margin  in  favor  of 
water-power  to  justify  this  auxiliary  use  of  steam,  imless  the 
latter  would  pay  on  its  own  account,  as  for  instance  in  a  plant 
used  largely  for  lighting,  which  would  be  the  most  profitable 
kind  of  electric  service  were  there  a  sufficiently  large  market. 
A  large  lighting  load  increases  the  peak  considerably,  but,  as 
compensation,  drops  the  peak  notably  during  summer  when 
low  water  generally  comes.  Particularly  is  this  the  case  with 
pubUc  lighting  which  in  summer  does  not  overlap  the  motor 
load. 

The  fundamental  questions  to  be  asked  in  taking  up  the 
supplementary  steam  plant  are  first,  how  large  a  plant  is  it 
advisable  to  install,  and  second,  how  much  energy  will  it  have 
to  contribute  to  the  common  stock.  To  determine  the  answers, 
the  distribution  of  flow  must  be  pretty  closely  kno\vn.  The 
hydraulic  power  may  fail  either  by  drought  or  by  flood.  If 
the  former,  there  is  likely  to  be  a  period  of  a  couple  of  months 


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HYDRAULIC  DEVELOPMENT. 


413 


in  which  the  power  will  be  subnormal,  perhaps  half  to  two- 
thirds  of  the  average  supply.  To  carry  a  full  load  over  this 
period,  implies  a  steam  plant  of  say  half  the  full  capacity  of  the 
hydraulic  plant,  in  operation  during  a  portion  of  the  time  for 
two  months.  To  put  things  on  a  concrete  basis,  suppose  a 
2,000  HP  hydraulic  plant,  with  a  1,000  HP  supplementary 
steam  plant.  As  stations  ordinarily  run,  the  load  for  a  con- 
siderable part  of  the  day  is  much  less  than  the  maximum  load. 
Fig.  231  shows  the  actual  load  curve  for  three  successive  days 
on  a  high  voltage  transmission  plant  doing  a  mixed  power  and 

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Fig.  231. 


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lighting  business.  It  shows  a  load  factor  of  very  nearly  50  per 
cent,  and  if  the  peak  of  the  load  corresponded  to  the  full  capacity 
of  the  hydraulic  plant,  during  the  period  of  low  water  under 
oiu*  assumed  conditions,  the  steam  plant  would  have  to  fur- 
nish, not  one-half  the  full  capacity  of  the  plant,  but  merely 
the  energy  above  the  half  load  ordinate  of  the  load  diagram. 
Considering  this  portion  of  Fig.  231,  it  will  be  seen  that  while 
at  the  peak  of  the  load  the  steam  plant  would  be  working  at 
full  capacity,  it  would  have  to  be  in  use  only  about  half  the 
time  at  a  load  factor  again  of  about  50  per  cent.  So  it  appears 
that  for  about  two  months  the  1,000  HP  steam  plant  will  be 
called  upon  for  only  about  one-fourth  of  the  actual  energy 


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414  ELECTRIC  TRANSMISSION  OP  POWER. 

delivered  by  the  plant,  say  between  four  and  five  per  cent  of 
the  year's  output. 

Therefore,  the  economic  question  is  the  cost  of  furnishing 
5  per  cent  of  the  yearly  output  of  a  steam  plant  of  half  the 
total  hydraulic  capacity  worked  at  about  half  load.  This  is 
a  very  different  case  from  any  ordinary  estimate  on  the  cost 
of  steam-power. 

Taking  the  cost  of  the  supplementary  steam  and  electric 
plant  in  this  case  at  $60,000  and  counting  interest,  depreciation, 
and  other  fixed  charges  at  10  per  cent,  there  is  an  annual 
charge  of  $6,000  against  the  plant  even  if  it  be  not  nm  at  all. 
At  the  assumed  load  conditions  it  will  cost  at  least  $25  per  day 
for  fuel,  supplies,  and  extra  labor  during  the  two  months  of 
low  water  so  that  the  upshot  of  the  matter  is,  that  it  will  cost 
not  less  than  $7,500  yearly  to  raise  the  limit  of  capacity  of 
the  plant  from  1,000  HP  to  2,000  HP. 

But  since  the  plant  load  factor  is  50  per  cent,  the  average 
output  is  raised  only  from  500  to  1,000  HP.  Even  on  this 
basis  the  supplementary  plant  evidently  will  pay  at  the  ordi- 
nary prices  of  fuel  and  of  electric  energy,  but  it  is  equally 
clear  that  as  the  required  supplementary  plant  grows  rela- 
tively larger,  and  the  proportion  of  steam  generated  output 
increases,  a  point  will  soon  be  reached  at  which  the  added 
annual  cost  cannot  be  compensated  by  increased  possible 
sales  of  power;  provided  of  course,  that  the  price  of  power 
sold  is  below  that  at  which  it  could  profitably  be  generated 
by  steam  alone  on  a  similar  scale. 

To  look  at  the  matter  from  another  side,  in  the  case  we 
have  been  considering,  the  total  effect  of  supplying  part  of 
the  output  by  steam  would  probably  be  to  increase  the  cost  of 
the  year's  output  by  less  than  15  per  cent,  and  the  supplemen- 
tary plant  would  pay  handsomely.  Probably  in  most  cases 
it  would  still  be  profitable  even  if  10  per  cent  of  the  total  out- 
put were  due  to  steam.  As  this  proportion  increases,  the 
advantage  diminishes,  and  finally  a  point  is  reached  at  which 
any  further  use  of  steam  cuts  down  profits  rapidly.  Each 
case  must  be  worked  out  by  itself,  as  the  result  depends  upon 
local  conditions,  and  generalizations  are  therefore  unsafe. 
It  is  usually  the  fact  however  that  a  stream  can  be  developed 


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HYDRAULIC  DEVELOPMENT.  415 

for  its  flow  in  early  sunfimer  with  entire  safety,  leaving  the 
minimum  flow  to  be  cared  for  by  a  supplementary  plant. 

In  a  few  instances  it  is  practicable  to  connect  the  power 
station  generators  so  as  to  be  operated  either  by  steam  or  by 
water-power,  thus  saving  the  cost  of  extra  generators.  But 
it  is  generally  better  to  place  the  supplementary  plant  in  a 
substation  at  the  receiving  end  of  the  line,  thus  allowing  it 
to  serve  as  an  auxiliary  plant  in  case  of  accident  to  the  line  or 
needed  repairs  to  the  hydraulic  works.  Turbo  generators  from 
their  economy  of  space  and  their  readiness  for  operation  are 
well  adapted  for  use  in  supplementary  plants,  and  for  such 
use  it  does  not  pay  to  install  a  costly  boiler  plant. 

Steam-power  on  a  12  hour  basis  at  steady  full  load  varies 
according  to  the  size  and  kuid  of  plant,  cost  of  fuel,  and  so 
forth,  from  a  little  under  $20  per  HP  year  to  $125  or  more, 
with  an  increase  of  one-third  to  one-half  in  case  of  variable 
loads.  Water-power  fully  developed  rents  for  from  $5  to  $50 
or  more  per  HP  year,  and  may  cost  to  develop  anywhere  from 
$20  to  $150  per  HP.  At  the  former  price  it  is  cheaper  than 
steam  under  any  circumstances ;  at  the  latter  it  is  dearer  than 
steam  unless  the  fuel  cost  is  abnormally  high. 

If  the  cost  of  hydraulic  development  can  be  kept  below 
$100  per  HP,  water-pc^wer  can  nearly  always  drive  steam-power 
out  of  business. 

With  respect  to  the  prime  movers  to  be  employed  in  a 
hydraulic  development,  one  must  be  governed  largely  by  cir- 
cumstances. The  choice  in  general  lies  between  turbines  and 
impulse  wheels,  the  properties  of  which  have  been  fully  dis- 
cussed in  Chapter  IX.  Without  attempting  to  draw  any  hard 
and  fast  lines,  turbines  are  preferable  up  to  about  100  feet  head, 
unless  very  low  rotative  speed  is  desirable,  or  very  little  power 
is  to  be  develop)ed.  Above  that,  the  impulse  wheels  grow 
more  and  more  desirable,  and  above  200  feet  head  the  field  is 
practically  their  o^n.  It  is  generally  practicable  and  desirable 
to  use  wheels  with  a  horizontal  axis.  Only  in  a  few  instances  is 
it  necessary  to  resort  to  a  vertical  axis,  as  when  there  is  consid- 
erable danger  of  the  tail-water  rising  clear  up  to  the  wheels,  or 
when,  as  at  Niagara,  a  very  deep  wheel  pit  is  employed. 

The  line  of  operations  in  developing  a  water-power  subse- 


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416  ELECTRIC  TRANSMISSION  OF  POWER. 

quent  to  the  reconnoissance  has  already  been  indicated.  After 
the  more  general  considerations  have  been  determined,  comes 
the  question  of  utilization. 

It  may  seem  needless  to  suggest  that  the  first  thing  neces- 
sary is  an  actually  available  market,  but  the  author  has  more 
than  once  had  imparted  to  him,  under  solenm  pledge  of  secrecy, 
the  location  of  "magnificent"  water-powers  which  could  be 
developed  for  a  mere  song,  located  a  hundred  miles  from 
nowhere — out  of  effective  range  even  of  electrical  transmission. 

Having  a  possible  market,  the  next  thing  is  to  investigate 
it  thoroughly.  The  actual  amount  of  st«am-power  must  be 
found,  together  with  its  approximate  cost  in  large  and  in 
small  imits.  This  information  ought  to  be  extended  to  at 
least  an  approximate  list  of  every  engine  used  and  the  nature 
of  its  use,  whether  for  constant  or  variable  load,  whether  in 
use  throughout  the  year  or  only  at  certain  seasons.  These 
more  mmute  data  are  not  immediately  necessary,  but  are 
inmiensely  useful  later.  If  it  is  proposed  to  include  electric 
lighting  in  the  scheme,  an  estimate  of  the  probable  demand 
for  lights  should  be  carefully  made.  A  fair  guess  at  this  can 
be  made  from  the  number  of  inhabitants  in  the  city  or  town 
supplied.  Where  there  is  comp)etition  only  with  gas,  experi- 
ence shows  that  the  total  number  of  incandescent  lights  installed 
is  likely  to  be,  roughly,  from  one-fourth  to  one-sixth  of  the 
population,  occasionally  as  many  as  one-third,  or  as  few  as 
one-eighth.  In  cities  of  moderate  size  it  is  usually  foimd  that 
even  with  competition  fn)m  gas,  the  annual  sales  of  electricity 
for  all  purposes  can  with  proper  exploitation  be  brought  up  to 
from  $1.50  to  $2.00  per  capita.  This  amoimt  may  be  increased 
by  50  per  cent  under  favorable  conditions. 

From  the  data  thus  obtained  one  can  estimate  the  general 
size  of  the  market,  and  hence  the  approximate  possible  demand 
for  electrical  energy.  With  this  in  mind,  further  plans  for  the 
hydraulic  development  can  be  made.  It  may  be  that  the 
water-power  is  obviously  too  small  to  fill  the  market,  if  so,  it 
shoulil  be  developed  completely.  If  not,  much  judgment  is 
necessary  in  determining  the  desirable  extent  of  the  develop- 
ment. Probable  growth  must  be  taken  into  account,  but  it 
cannot    safely   be    counted    upon.      If   steam-power  is   very 


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HYDRA  ULIC  DEVELOPMENT.  417 

expensive  most  of  the  engines  can  probably  be  replaced  by 
motors.  The  replacement  of  one-half  of  them  is,  under 
average  circumstances,  a  sufficiently  good  tentative  estimate. 

With  this  as  a  basis,  approximate  estimates  of  the  hydraulic 
development  can  be  made.  This  should  be  done  by  a  compe- 
tent hydraulic  engineer.  If  the  developemnt  is  easy  it  is  well 
to  make  estimates  for  a  liberal  surplus  power  also.  At  this 
stage  it  is  best  to  have  the  hydraulic  and  the  electrical  engi- 
neer work  hand  in  hand  to  estimate  on  the  delivery  of  the 
assumed  amount  of  power.  From  these  estimates  the  general 
outlook  for  returns  can  be  reckoned. 

Before  actually  beginning  work  it  is  advisable  to  make  a 
pretty  thorough  preliminary  canvass  of  the  market,  to  see 
what  can  be  done  immediately  in  the  sale  of  power  and  light. 
With  the  certain  and  the  probable  consumption  ascertained, 
the  hydraulic  and  electrical  engineers  can  work  their  plans 
into  final  shape  and  prepare  final  estimates. 

All  this  preliminary  work  may  at  first  sight  seem  rather 
unnecessarily  exhaustive,  but  mistakes  on  paper  are  corrected 
more  easily  than  any  others,  and  the  investigation  is  likely  to 
save  many  times  its  cost  in  the  final  result. 

Whatever  is  done  should  be  done  thoroughly.  Poor  work 
seldom  pays  anywhere,  least  of  all  in  a  permanent  installation, 
and  it  should  be  conscientiously  avoided. 

Above  all,  continuity  of  service  has  a  commercial  value  that 
cannot  be  estimated  from  price  lists.  If  it  anywhere  pays  to 
be  extravagant,  it  is  in  taking  extreme  precautions  against 
breakdowns  and  in  facilities  for  quick  and  easy  repairs  in  case 
of  unavoidable  accident.  This  applies  alike  to  the  hydraulic 
and  the  electrical  work.  If  the  first  severe  freshet  demoral- 
izes the  hydraulic  arrangements,  or  the  plant  runs  short  of 
water  at  the  first  severe  drought,  a  damage  is  done  that  it 
takes  long  to  repair  in  the  public  mind.  On  the  other  hand, 
careful,  thorough  work,  coupled  with  intelligent  foresight, 
insures  that  complete  reliability  that  is  the  mint  mark  of 
honest  and  substantial  enterprises. 


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CHAPTER  XI. 

THE   ORGANIZATION   OF   A    POWER   STATION. 

The  first  thing  to  be  determined  in  planning  a  power  station 
is  the  prop)er  site,  which  should,  if  steam  be  the  motive  power, 
be  settled  by  convenience  with  respect  to  the  supply  of  coal 
and  water.  In  using  water-power  the  position  of  the  station 
should  be  determined  in  connection  with  the  hydraulic  devel- 
opment. Near  the  foot  of  the  working  fall  is  the  natural  site, 
but,  particularly  in  mountainous  regions,  it  may  be  quite  im- 
practicable on  account  of  lack  of  available  space,  unsuitable 
ground  for  foundations,  inaccessibility,  or  more  often  danger 
of  flood.  Under  high  heads  where  a  pipe  line  is  used,  one 
has  a  considerable  amount  of  freedom  in  determining  the  site, 
since  the  pipe  can  be  extended  and  led  around  to  convenient 
locations  at  moderate  expense,  say  not  more  than  $3  or  $4  per 
foot.  A  relatively  small  sacrifice  of  head,  too,  may  enable  one 
to  secure  an  admirable  location. 

On  low  heads  there  is  far  less  latitude  permissible,  since  the 
canal  and  tail-race  are  relatively  costly,  and  a  change  of  level 
is  a  serious  matter. 

The  proper  location  and  design  of  a  power  house  calls  for 
great  tact  and  judgment.  Often  hampered  by  the  topographi- 
cal conditions,  the  site  selected  must  be  such  as  to  secure  good 
operative  conditions  at  minimum  cost.  It  is  well  in  approach- 
ing the  subject  to  put  aside  all  preconceived  notions  as  to 
how  a  plant  should  look,  and  to  remember  that  it  is  a  strictly 
utiUtarian  structure.  On  the  hydraulic  side  it  should  have 
easy  access  and  exit  of  the  water  with  the  minimum  loss  of 
head,  the  shortest  feasible  penstocks,  and  the  greatest  security 
from  variations  of  head.  On  the  electrical  side  it  must  be 
dry  and  clear  of  floods,  conveniently  arranged  for  all  the  appa- 
ratus, and  with  an  easy  entrance  for  the  transmission  lines. 
Withal  it  must  have  solid  foimdations,  must  often  be  capable 
of  easy  future  extensions  and  must  meet  all  these  require- 
ments at  the  minimum  expense. 

418 


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THE  ORGANIZATION  OP  A  POWER  STATION.       419 

Some  of  these  conditions  tend  to  be  mutually  exclusive. 
When  a  plant  is  built  at  once  to  the  full  capacity  of  the  hydrau- 
lic privilege,  the  conditions  are  considerably  simplified,  but 
this  is  not  the  usual  case.  The  plants  incidentally  described 
in  this  chapter  have  been  chosen  as  illustrative  of  some  of  the 
problems  of  power-house  organization  rather  than  as  models 
of  any  recognized  canons  of  design.  There  are  no  such,  save 
in  the  very  general  way  already  indicated,  and  few  of  the 
plants  erected  are  not  in  some  particular  open  to  severe  criti- 
cism. 

But  their  design  has  been  necessarily  a  compromise,  and 
more  often  than  not,  the  objectionable  features  have  resulted 
from  following  some  fashion  set  by  some  conspicuous  plant 
working  imder  different  conditions.  The  best  watchwords  in 
power-house  design  are,  safety,  operative  simplicity,  and 
accessibiUty.  Heeding  these,  with  a  keen  eye  to  local  peculi- 
arities one  is  not  likely  to  go  far  astray. 

If  possible,  the  power  station  should  be  placed  well  off  the 
main  line  of  flow,  or  with  the  main  floor  well  above  high  water 
mark.  The  foimdations  must  be  of  the  best  to  secure  safety 
from  floods  and  a  proper  support  for  the  moving  machinery. 
To  meet  these  conditions  is  not  always  easy,  particularly 
when  the  available  head  is  low,  and  sometimes  extreme  artificial 
precautions  have  to  be  taken  against  flood.  Such  a  case  is 
found  in  the  Oregon  City  plant  already  mentioned,  of  which  a 
sectional  view  is  given  in  Fig.  232,  showing  the  foundations,  a 
single  generator,  its  wheels,  and  their  appurtenances.  The 
inner  wall  of  the  station  is  here  the  outer  wall  of  the  canal, 
and  both  walls  and  foundations  are  built  very  solidly  of 
masonry  and  concrete.  In  the  cut  A  and  B  are  the  draft 
tubes  belonging  respectively  to  the  wheel  cases  D  and  F,  which 
are  supplied  by  the  penstocks  C  and  E.  F  contains  the  regu- 
lar service  turbine,  a  42  inches  Victor  wheel  coupled  direct  to 
the  generator  at  P.  On  the  pedestals  G  above  this  wheel  is  a 
ring  thrust  bearing  at  /  and  an  hydraulic  thrust  bearing  K. 
Above  this  is  a  pulley  y,  6  feet  in  diameter,  and  still  above 
this  the  upper  bearing  support,  the  bearings  N  and  0,  the 
coupling  M,  and  pedestals  Q. 

The  wheel  case  D  contains  a  60  inch  wheel  with  bearings,  pul-* 


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420 


ELECTRIC  TRANSMISSION  OF  POWER. 


iki.  'JS^I, 


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THE  ORGANIZATION  OF  A  POWER  STATION.       421 

ley,  and  so  forth,  R,  S,  W,  T,  U,  The  function  of  this  wheel  and 
its  attachments  is  to  supply  power  at  the  seasons  of  very  high 
water,  sometimes  several  years  apart.  When  the  tail-water 
backs  up  so  far  that  the  smaller  wheel  is  no  longer  equal  to  the 
work,  the  generator  shaft  is  arranged  to  be  uncoupled  just 
above  the  wheel.  Then  the  belt  tightener  X  can  be  brought 
into  use,  the  large  wheel  started,  and  the  generator  driven  by 
the  horizontal  belt.  The  belt  tightener  is  operated  by  hand 
wheels  at  E2  and  Dj,  while  similar  hand  wheels  at  C^  and  B^ 
enable  the  wheels  to  be  regulated  by  hand  when  desirable. 
The  governing  is  normally  accomplished  by  the  automatic 
regulator  A2.  F^  is  one  of  the  main  race  gates,  lifted  by  the 
mechanism  at  Gj.  The  wheel  room  is  Hghted  by  water-tight 
heavy  glass  bulls  eyes  at  Z,  each  three  feet  in  diameter.  The 
d^Tiamo  room  is  lighted  by  side  windows  and  monitor  roof,  and 
is  fitted  with  a  twelve  ton  travelling  crane  K^j  carried  on  the 
supporting  column  M^  and  N^.  The  penstocks  pass  through 
the  hea\y  cement  floor  of  the  wheel  room,  J^j  with  water-tight 
joints.  The  main  point  of  interest  in  this  station  for  our  pres- 
ent purpose  is  not  the  very  complicated  and  cumbersome 
hydraulic  plant  but  the  structure  of  the  wheel  room,  which 
forms  a  massive  permanent  coffer  dam  securing  the  motive 
power  against  all  direct  interference  by  even  the  fiercest 
floods.  Such  a  construction  is  somewhat  inconvenient,  but  in 
some  instances  is  almost  absolutely  necessary.  The  design  of 
this  plant  is  imique,  in  some  respects  uniquely  bad  from  the 
standpoint  of  general  practice,  but  many  of  its  peculiarities  are 
the  result  of  its  situation  and  of  imusual  conditions  of  water 
supply  which  forced  the  use  of  uncommon  remedies.  Generally 
such  extreme  measures  need  not  be  taken,  although  since  it  is 
usually  desirable  to  have  the  dynamos  on  a  level  with  the 
wheels,  and  coupled  to  them,  a  water-tight  wall  between  the 
d3aiamo  room  and  the  wheel  room  is  rather  common.  Quite 
as  often,  however,  full  reliance  is  placed  on  the  strength  and 
tightness  of  the  penstocks  and  wheel  cases,  and  wheels  and 
djoiamos  are  placed  in  the  same  room.  A  plant  so  arranged 
is  cheap  and  simple,  and  where  there  is  no  unusual  danger 
of  flood  is  sufficiently  secure.  Fig.  233  shows  a  good  typical 
plant  of  this  sort,  consisting  of  three  double  horizontal  tur- 


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422 


ELECTRIC  TRANSMISSION  OF  POWER. 


bines  under  50  feet  head,  each  directly  coupled  to  its  generator. 
Each  pair  of  wheels  gives  560  HP  at  about  430  revolutions  per 
minute.  This  represents  construction  as  straightforward  and 
simple  as  that  of  Fig.  232  was  difficult  and  intricate.  It  is 
specially  interesting  in  the  arrangement  of  several  wheels  to 
discharge   into   a   common   tail-race,  instead  of  into  several 


Fig.  233. 

costly  arched  tail-races  extending  imder  the  dynamo  room,  a 
construction  sometimes  quit€  unnecessarily  employed. 

The  hydraulic  conditions  may  drive  the  engineer  to  all  sorts 
of  expedients,  but  the  main  points  are  security  against  being 
drowned  out,  and  good  foundations.  If  the  dynamos  and 
wheels  can  be  given  direct  foundations  of  masonry  and  con- 
crete, such  as  the  former  have  in  Fig.  233  and  the  latter  in  Fig. 
232,  so  much  the  better.     If  moving  machinery  must  be  carried 


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THE  ORGANIZATION  OF  A  POWER  STATION. 


423 


on  beams,  support  these  beams  as  in  Fig.  233,  directly  mider  the 
load,  J)y  iron  pillars  or  masonry  piers.  For  direct  coupling  it 
is  preferable  to  have  foundations  entirely  secure  from  vibration.